Ubuntu 20.04 unable to remove proposed

Just add an octothorpe to the start of the line and the repository will be disabled. You will need to run the editor as root.

When you run software-properties-gtk you have to switch to the "developer options" tab to find the setting to enable/disable -proposed.

as I said in my initial question i'm unable to deactivate proposed by 'software and updates' aka software-properties-gtk.
flipping the option to enable/disable -proposed adds/removes the 'proposed' line in /etc/apt/sources.list but does not remove the /etc/apt/sources.list.d/proposed.list. Obviously i can delete /etc/apt/sources.list.d/proposed.list to stop the problem but i want report the bug to the developers so they can correct the daily ISO.

IMPORTANT: I don't have the same problem installing from the release ISO Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)

So I repeat my initial question: how to open a bug for this problem?

ubuntu- bug apt

may be correct?

If you edit the text file does it work OK. Ignore the GUI app, we know it doesn't work.

According to https://iso.qa.ubuntu.com/qatracker/milestones/408/builds the target for a bug with the daily live ISO would be either ubiquity or the affected package (which would probably be apt as already suggested by you).

I just dowloaded the file
http://www.cdimage.ubuntu.com/focal/daily-live/20200622/focal-desktop-amd64.iso
and I can confirm that it contains a file /casper/filesystem.squashfs/etc/apt/sources.list.d/proposed.list with contents
deb http://archive.ubuntu.com/ubuntu/ focal-proposed main restricted

Also the focal daily live from 2020-06-23
http://www.cdimage.ubuntu.com/focal/daily-live/20200623/focal-desktop-amd64.iso
has such file.

The release iso for focal
http://releases.ubuntu.com/focal/ubuntu-20.04-desktop-amd64.iso
does not have such file.

(I did not have time to investigate the consequences of the existence of that file on the installer ISO)

Nice find Manfred :-)

With this in mind, Corrado did you install from the daily ISO before Focal was released?

Did you read my 1st post? I wrote:
Installed Ubuntu 20.04 from daily ISO Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200622)
on my calendar 20200622 = June 22 is about 1 month AFTER Focal released
and I noted also:
I don't have the same problem installing from the release ISO Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
So the new ISO are WRONG and I should open a bug.... for APT?

opened the related bug: https://bugs.launchpad.net/ubuntu/+bug/1884937

See https://bugs.launchpad.net/bugs/1879373 which explains why -proposed is enabled on post release ISOs.

No. The new ISO is right. You just left proposed enabled after using the daily build. This is normal.