23.04 installer not showing the lvm partitions under luks

This is definitely an issue and I don't think you're doing anything wrong. I just booted from the released version and went through my usual and well documented process and it is not working. I booted from a 22.04 USB and checked and the process still works as expected there, so this is definitely a problem with the installer.

It's disappointing that it still doesn't support proper LUKS encryption natively, but this is definitely worse in that you can't even do it manually the way you used to be able to with the old installer.

Might be a bug, or maybe purposely left out of the new installer.
There is a note somewhere about it, like some functions have been left out like the zfs, and also there is a link there that contains an iso with the legacy installer so that might sort out the problem of lvm/luks combo .

During the install look for the Advanced features button for LVM it is within the installer.
Click on the 'Advanced features...' button
    The 'Advanced Features' dialog is displayed
Select 'Use LVM with the new Ubuntu installation' and check 'Encrypt the new Ubuntu installation for security'
    'Encrypt the new Ubuntu Installation for security' and 'Use LVM with the new Ubuntu Installation' is checked
Click on the 'OK' button
    The dialog closes and 'LVM and encryption selected' is displayed next to the 'Advanced features...' button
Click on the 'Install Now' button
    The 'Choose a security key' screen is displayed with the 'Install Now' button greyed out
Enter a security key and type a different key into the 'confirm the security key' input box
    The installer displays 'Passwords do not match'
Enter a security key and type the same key into the 'Confirm the security key' input box
    The passwords are accepted and the 'Install Now' button can be clicked
Check 'Overwrite empty disk space'
Click on the Continue button (if there is only one hard disk in the system, the button should read 'Install Now')
    'Write the changes to disks' dialogue appears
Click Continue
If there is only one hard disk, skip to step 18 (On the 'Where are you?' screen...). Otherwise, on the 'Installation type' screen verify that the drive selected on the Select drive list corresponds to the drive on the chart (e.g /dev/sda)
    Selected drive is displayed on the chart
Verify that the full drive space is allocated
    Full drive space is allocated for installation
Click on the Install Now button

Thank you Bernard Stafford for your time and effort to write an answer what you believe might be the answer I was looking for, but I think you answered a different question.

But I have found this:
https://ubuntu.com/blog/ubuntu-desktop-23-04-release-roundup
and this states that:
"Some of the features available in the legacy installer have not yet made it across to Subiquity and support for features like ZFS guided install will be included in Ubuntu 23.10. In the meantime, users who wish to take advantage of this feature can download an alternative Ubuntu Desktop 23.04 ISO that uses the legacy installer."

Therefore

Someone has to say that: "This function you are looking for is not included in the new installer" and that I can mark as solved with the button :" This Solved My Problem", since I cannot mark my own comment as such.

Thank you all for the kind attentions and helpful attitude!

(edit: oh yeah, there is a "Problem Solved" button below, I think that will do too!)
(edit 2: its says, "there is 1 error", well ok, I will click on I still need an answer :)
(edit 3: it says again "There is 1 error", so I have to wait for someone to comment that I have commented a few lines above)

That note just says "like ZFS guided install" -- so we're making a bit of a leap here that this is intentional. I would definitely suggest we should look for an official response that Canonical thought full-disk encryption support wasn't important to include in the new installer for their 2023 release of Ubuntu.

So to summarise, the best solution/workaround is to download the ISO image with the legacy installer, correct?

PS: In my rush, I rebooted without closing the LUKS volumes, and then I initially wasn't able to get back into my currently installed system, so beware! (Yes, I'm a total n00b). Just thought I'd provide some example commands for those using LUKS + LVM on how to properly close the volumes:

vgchange -a n <vg_name>
cryptsetup close <crypt_partition_name>

@Mark Bainter
it says: "Some of the features available in the legacy installer have not yet made it across to Subiquity and support for features like ZFS guided install will be included in Ubuntu 23.10. "

or

Some of the features......

That means "....like zfs guided install.... >>amongst other things<<" that is still available in the legacy installer

with the legacy installer I managed to install as I wanted, I didnt test for zfs tho, since I have no interest in it as of yet.

@Shahid N.
Yes, I think so, for now.

I am one of the users affected by this limitation.
Request other affected users to upvote the following issues -

1. https://github.com/canonical/ubuntu-desktop-installer/issues/1770
2. https://github.com/canonical/ubuntu-desktop-installer/issues/1353