I remember seeing a discussion somewhere on Ubuntu Forums about the fact that the installer for 24.04 no longer has the option for manually encrypting partitions.
Choosing to erase and use the entire disk with encryption obviously still works.
Sorry, was not able to find the thread.
However, there is a way to encrypt partitions post-install which is discussed here.
Obviously, make sure you have excellent backups before attempting this and also my caveat which is that I have not tried this method.
You do so at your own risk.