Hide a user account from the login screen of macOS Catalina
The trick still works in Catalina:


Log out as admin (below "admin" is meant as name of the admin user account - adapt accordingly) and login as foo.
Open the Terminal and enter:

su admin
sudo dscl . create /Users/admin IsHidden 1

If you want to remove the login screen's "Others..." option, which appears after disabling the admin account, enter:

sudo defaults write /Library/Preferences/com.apple.loginwindow SHOWOTHERUSERS_MANAGED -bool FALSE

To reverse the hidden admin user and enable "Others..." again enter:

su admin
sudo dscl . create /Users/admin IsHidden 0
sudo defaults delete /Library/Preferences/com.apple.loginwindow SHOWOTHERUSERS_MANAGED





Tested in a Catalina VM. This might not work for T2-equipped Macs and definitely not if FileVault is enabled. 



To remove the user admin from the login screen on a filevaulted Mac you can simply remove this account from the FileVault enabled users:


Log out as admin and login as foo
Open the Terminal and enter:

su admin


List all users to be sure that user admin and foo are FV enabled:

sudo fdesetup list
sudo fdesetup remove -user admin


After removing admin only one user is left to unlock the system volume!
To remove the user admin from the intermediate login screen (i.e. volume still unlocked and after logging out as foo)

su admin
sudo dscl . create /Users/admin IsHidden 1
sudo defaults delete /Library/Preferences/com.apple.loginwindow SHOWOTHERUSERS_MANAGED



Use at your own risk! Tested in a Catalina VM. This might not work for T2-equipped Macs.