I think that:
sudo apt purge xfce4*
sudo apt autoremove
should do it.
I’ll point out that the other answers here are also correct. It depends on how you want to clean it from your system.
“apt remove” will only remove the packages, not the config files
“apt purge” will remove the packages and config files
“apt autoremove” will clean up the orphaned dependencies
“xfce4” will only remove the DE
“xfce4*” will remove the DE and most of the other packages that come with xfce
Not every package that comes with xfce has a name that starts with xfce4, just most of them do. You may need to identify and remove other packages too.
That's the way. However, I'd like to add that when I used Debian, I regularly got leftovers after uninstalling things, especially when removing big things work lots of dependencies. So expect some dependencies to remain.
By uninstalling the corresponding packages, I guess ... Don't know in detail how Debian works nowadays, but there will likely a meta package or a package group you can remove to remove Xfce with all dependencies
You can run sudo apt purge xfce4 (if you wan to remove all of its components and dependencies) or sudo apt remove xfce4.
<br>
If you don't have an other DE installed, I suggest you find one and do it form the other DE, because if you don't, you're gonna end up without a graphical environement, and you would have to do everything frop the terminal.