But sure, things could be done to make this more difficult. It's probably not worthwhile on Reddit's part to do anything to stop this, just as it hasn't been too worthwhile for websites to evade ad blockers. The number of people who mass delete is just too small to matter.
If I worked at Reddit and wanted to do something about it though (and was a programmer), I'd add an option under individual deleted comments for viewers to click to view the comment (and any versions). And possibly add an option for viewers to restore a version entirely. This would save helpful comments, at least until some jerk decided to automate the process and restore everything. So maybe the complete restoration is a bad idea.
That could still backfire. Users may be very unhappy that their unedited comments are accessible forever. This may drive them away from commenting and participating in general.
The goal of mass-deleters is to drive down engagement. If Reddit makes the entire edit history of each comment accessible, then mass deleters could flood that history with bogus, AI-generated crap. Although it may still be possible to determine which edit was the last real one, the effort to do so goes way up, and engagement goes down as a result.
If they retain versioning history I'm sure it would be easy to identify a mass edit and revert all of those edits from the user. If it wasn't easy, for some reason, it would probably be easy to revert all edits after, say, 2 days of posting.
Given that everything posted to Reddit becomes the property of Reddit (okay, perpetually licensed to Reddit), I don't know that much legally could be done about this. Unless they restored stuff posted while under-age, or PII, maybe.