Leaves behind a bunch of stuff. You have to manually remove each Snap individually, plus the snapshots they take and then hide, and then use Snap to remove itself (it doesn’t let you), then you can apt purge snapd.
There’s several levels of “we know better than you so we’ll just keep this here for when you inevitably change your mind” and it is exhausting.
I don’t even know if the above would also clean up all the dev/loop cruft. It was an unpleasant surprise to discover that apt remove alone didn’t at least disable all the systemd .mount units.