Anyway, we could also have an special email address that would
automatically create bugs under a dedicated account. But the user would
have no way to edit it afterwards.
Never mind editing, that unregistered use would have no way to participate in the discussion.
What were could do is to have the server of that special email address also forward and relay communications to and from the user to the bugs that he created or that he participated in. IOW change the backend of the current debbugs email addresses.
In principle (I admit I haven't fully thought this through) this should coexist nicely with the interactions from reagistered Gitlab users, developers or not.
Just a thought,