--purge does not do anything with dependencies. You will need to explicitly remove all packages that are marked as installed manually, i. e. all packages that you pointed a package manager to install. If a DE was installed automatically by Debian installer, or if you installed it with apt install xfce4, the only manually installed component it the xfce4 metapackage, and using the --auto-remove flag will remove all its dependencies. But if you additionally installed any components or packages that depend on that components, you will also need to clean them up manually.
That’s just how package managers seem to work in my experience. Even using
--purgeon APT leaves behind a ton of junk.--purgedoes not do anything with dependencies. You will need to explicitly remove all packages that are marked as installed manually, i. e. all packages that you pointed a package manager to install. If a DE was installed automatically by Debian installer, or if you installed it withapt install xfce4, the only manually installed component it thexfce4metapackage, and using the--auto-removeflag will remove all its dependencies. But if you additionally installed any components or packages that depend on that components, you will also need to clean them up manually.