I’m not on Debian, but a quick search led me to this wiki link from Arch. Give it a whirl:
https://wiki.archlinux.org/title/GDM#Hide_user_from_login_list
Edit: context of the Arch wiki link, in case better answers can be gleaned from it, I found it on AskUbuntu: https://askubuntu.com/questions/2471/how-to-hide-users-from-the-gdm-login-screen
Ref: https://forum.f-droid.org/t/fennec-vulnerability-recommended-to-uninstall/