Selected PHP Version in Jail
Taken from /etc/jailkit/jk_init.ini:
# Debian 10 default php version is 7.3 (Debian 9 is 7.0) # Todo: set default version in ISPConfig installer, # but install the php cli version matching the website
In this case, should switching the PHP version remove the old PHP version from the jail? To me it looks like that would be hard to implement, considering ISPConfig doesn't remove redundant things (aka sections or applications I removed from System > Server Config > Jailkit that were previously there) from jails after re-syncing shell users.
If this is the specific reason it wasn't implemented yet, I think an easier approach would be including all PHP versions in the jail, and just modify the php (no version number) binaries to be symlinked to the right version like
sudo update-alternatives --config php does (this command only works outside of the jail).