Why two packages from main repositore have support to April 2021 year?

I suggest you report a bug to make the package teams aware. This will get them updated to supported versions

For diagnostic purposes, what is the output of the command

apt-cache policy gstreamer1.0-gtk3 libbrotli1 libwoff1

I think that I have an explanation for libbrotli1 libwoff1:
At publication time of Ubuntu bionic these packages were in the "universe" category, but they meanwhile have been moved to "main".

Yes, you have right that these packages were in the "universe" category but why they are still support only to 2021 year if they now are in "main" repository? This mean that after April 2021 these packages will be not supported at all and I should remove it?

My personal assumption was that during the move of the packages from universal to main the support information was not correctly updated, and in reality they will be supported further.

Why do you see the support information as reason for uninstalling these packages? If you are so much focusing on support, then you consequently have to abandon using Lubuntu 18.04 in two weeks' time and switch to a different operating system with continued support.

oops, wrong autocorrect, should be "universe"

@Manfred Hampl, yes I will change operating system because this will end of support. I don't want to use not maintained operating system. This works perfectly but this is time to divorce because Ubuntu family don't want support LXDE packages. It is very sad :/ I very like this environment. It is very fast. I am very suprised and I love this operating system.

P.S. Information from ubuntu-support-status are updated? After April 2021 these packages will change date to 2023 year?

gstreamer1.0-gtk3 libbrotli1 libwoff1

I have also Ubuntu 18.04.5 on another computer so this is important question because after April 2021 if I will muse removed these packages maybe my system will be not work normally... What do you think?

"if I will muse removed these packages maybe my system will be not work normally.."

I am not sure if I correctly understand your question, but it is possible to run an Ubuntu system that has packages from "main" only and no package from restricted or universe or multiverse.

Manfred Hampl , are you 100% sure? I have installed Ubuntu 18.04.5 with default configuration without restrictions packages and additional formats (codec etc). I remember you that we talk about packages from main, not from universe:

" gstreamer1.0-gtk3 libbrotli1 libwoff1" - all packages are now on main repository and if I will remove this I think that this can cause crash of my system.

Why should removing a package crash your system?

You will have a system that does not have the package that you have removed, and all packages that directly or indirectly depend on it. This should never lead to a crash.

Okay, so I will try remove this if it will be necessary after end of support these packages. However I would like to know what is gstreamer1.0-gtk3. I think that this is needed for playing videos because I can see that this is used when I watch youtube videos. Have I right? If answer is yes this means that without this package I can't play videos, any videos.

" GStreamer is a streaming media framework, based on graphs of filters
 which operate on media data. Applications using this library can do
 anything from real-time sound processing to playing videos, and just
 about anything else media-related. "

Where did you find the information that support for gstreamer1.0-gtk3 ends now?
It is in "main", see e.g. https://packages.ubuntu.com/bionic-updates/gstreamer1.0-gtk3 and https://launchpad.net/ubuntu/+source/gst-plugins-good1.0

Mr Manfred Hampl, I used this command ubuntu-support-status --show-all

Now I notice that I don't have installed this package (gstreamer1.0-gtk3) anymore so this are two option.

This packages was removed in update but I don't think so.
Second option is that this package was installed but on my second machine with Ubuntu 18.04 instead Lubuntu 18.04

On Lubuntu 18.04 now I have only two packages from Canonical with support to April 2021:

Wsparcie do kwietnia 2021 (Canonical - 3y):
libbrotli1 libwoff1

Both packages are from main repository. You can check this with using apt show command.

It seems to me that you have a found an inconsistency in the package directory of Ubuntu.

If a package is moved from one repository component to another one (e.g. from universe to main), then apparently the information about the support period is not updated, and consequently the output of ubuntu-support-status is not as expected.

I can see for libwoff1 amd64 on bionic:

http://archive.ubuntu.com/ubuntu/dists/bionic/universe/binary-amd64/Packages.xz
contains

Package: libwoff1
Architecture: amd64
Version: 1.0.2-1
...
Section: universe/libs
...
Filename: pool/universe/w/woff2/libwoff1_1.0.2-1_amd64.deb
...
Supported: 3y

and
http://archive.ubuntu.com/ubuntu/dists/bionic-updates/main/binary-amd64/Packages.xz
contains

Package: libwoff1
Architecture: amd64
Version: 1.0.2-1build0.1
...
Section: libs
...
Filename: pool/main/w/woff2/libwoff1_1.0.2-1build0.1_amd64.deb
...
Supported: 3y <------ unexpected value!

Similar information is there also for gstreamer1.0-gtk3 and libbrotli1

I suggest that you try getting into contact with the developers, perhaps using the mailing list https://lists.ubuntu.com/mailman/listinfo/ubuntu-devel-discuss

Thanks you Manfred Hampl. I didn't use yet mailing list. Maybe here on launchpad I can report this somewhere? Do you know if it possible?

In my opinion the ubuntu-devel discussion list is the best address for reporting this. I do not have any other suggestion.
Eventually the #ubuntu-devel irc, see https://wiki.ubuntu.com/IRC/ChannelList

By the way, why is this so important?

The most important thing for me is have installed package which have support. I don't want to use not supported packages for security reasons so I must have sure that these packages are still maintained and I will get updates.

I created bug on launchpad and I sended e-mail to each maintainer of the packages but problem still exist.

@Manfred Hampl, I used this command today: apt-cache policy gstreamer1.0-gtk3 libbrotli1 libwoff1

I can see output something like this for gstreamer1.0-gtk3 :

Installed: 1.14.5-0ubuntu1~18.04.2
Candidate: 1.14.5-0ubuntu1~18.04.2

Table of version:

*** 1.14.5-0ubuntu1~18.04.2 500

500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
500 http://archive.ubuntu.com/ubuntu bionic-security/main amd64 Packages
100 /var/lib/dpkg/status

1.14.0-1ubuntu1 500

500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages

I readed on the internet that 500 priority means that package is not installed but this is not truth. I checked with apt list --installed and I have installed these packages. Can you explain me why I have 500 priority and what is mean that 100 priority for /var/lib/dpkg/status ???

"I readed on the internet that 500 priority means that package is not installed"

This is nonsense. Where did you read that? Please provide the link for requesting a correction.

The information whether a package is installed (or not installed) is in the first lines "Installed: 1.14.5-0ubuntu1~18.04.2" vs. "Installed: (none)"

The numbers are - as already written by you - a priority setting. If you request an installation of a package, then the version with the highest priority value is installed (if dependencies can be met).

For some additional details see https://help.ubuntu.com/community/PinningHowto

I used google and I readed this: "By default, each installed package version has a priority of 100 and a non-installed package has a priority of 500. The same package may have more than one version with a different priority. APT installs the version with higher priority unless the installed version is newer."

I didn't visit this website because it is unknown by me but I will share with link but I don't know if it website is safety and you must make decision if you are trust or not this website. Link: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwi62bWTlsrwAhXJHXcKHVzNCKcQFjACegQIBBAD&url=https%3A%2F%2Fitsfoss.com%2Fapt-cache-command%2F&usg=AOvVaw2wlWOVdOCG8xq-YwJLL0FV

You are misinterpreting the text.
It does not say "if it is 500 then it is not installed", but it says "if it is not installed then it usually has prio 500"

Did you read the documentation that I linked in comment #21 and the "further reading" mentioned at the bottom of that page?

Okay, thank you Manfred. Now I think that I understand more and I know that this option is not to check what package is installed.