In theory I think you'd still need the temp user as long as temp users might have different permissions from anonymous users (by default they don't but the system does offer the possibility of temp-user-specific permissions).

Change 1007648 had a related patch set uploaded (by Tchanders; author: Tchanders):

[mediawiki/core@master] DNM Experimental: Remove unsaved temp user from EditPage

https://gerrit.wikimedia.org/r/1007648

I wonder (though I'm on the fence still) if we could just make the decision that $wgGroupPermissions['*']['edit'] must be true in order for temp accounts to be created via edit. You could still give temp accounts other additional rights, but an anon user must make a successful edit (or whatever other action you've configured to create a temp user) first.

We're expecting other workflows to do this rather than handle unsaved/placeholder temp users (see our guidance about this).

The unsaved temp user is proving a bit odd, e.g. AbuseFilter logs the name, but it never gets mapped to a real user (T334623#9587082).