Desktop environment isn't working after trying to install GNOME 40
Removing repository with sudo apt-add-repository -r ppa:shemgp/gnome-40 will not downgrade the packages to their stock versions.
You have to use ppa-purge instead.
So you have to re-add the problematic PPA by
sudo apt-add-repository ppa:shemgp/gnome-40
then install ppa-purge by
sudo apt-get install ppa-purge
and finally revert all GNOME packages to their official versions using
sudo ppa-purge ppa:shemgp/gnome-40
Then reboot.