And just who are the "worst" offenders?
You really don't know or are you speaking in general? O_o
In general. For example, if you and I compared lists, we'd have several names in common, and quite probably a few that aren't in common. On the other hand, if you compared RWarehall's list with Vain's list...
By what measure do you identify them retroactively, especially considering all the edited and deleted posts even from the past year?
Something is still there (also PMs). (btw, I blame those geniuses who spam reported proofs)
Some of it may still be buried in the databases, sure. For that to be useful, someone would have to be able to find it. Also assuming, of course, that it was setup to keep the deleted posts in the first place instead of straight up discarding them.
Many of the worst posts were hit with deletion precisely because they were terrible. A self-moderating function working (more or less) as intended.
If we open up PMs for review, then you may have people getting banned for conversations that neither participant thought were a problem (like taking one's insults of a user out of the "public" sphere). Additionally, a great many PMs were lost during the shift to the PM functionality; granted, that was years(?) ago, but still.
There are at least some non-alt users that don't have a problem with the people generally considered to be trolls.
Sure, people who don't check the forum frequently won't notice anything wrong.
Remember the GG thread? Both sides claiming the other side is trolling, and both sides with people who frequent the forums. Generally each side was more tolerant of those on the same side.
It's much, much simpler (and in many ways fairer) to declare "Alright. Everyone is basically forgiven for past crimes during the era of non-enforcement. New policies start now and apply to everyone."
Because it's like admitting: "You've been smart to steal games and harass people in the past. Now you'll be considered equal to other goggers, even those who helped the community or made a lot of generous giveaways.
Everyone is given an equal footing to start out. Because the new rules have a hard deadline, and because habits don't change quickly, the serial offenders are very, very likely to rapidly incur penalties that nicer forumites will not. So more like "Here's your clean slate. Try to wait at least a day before you boop it up."
In the meantime, the rest of us who have been here are already aware of the problem makers, and we can take steps to avoid being scammed by them. Your forum reputation doesn't disappear just because the "legal reputation" (if you will) has been wiped clean one time; other people are required by the rules to be civil, but there is no requirement to interact with the historically annoying or include them in current and future giveaways.