As one of the authors of a non-joke answer that was deleted, I'm disappointed by the characterization that the rest of the answers on that question were useless or duplicates. I made six revisions to my answer to ensure it was somewhat detailed and accurate. Rather than simply saying, "do this," I described how I guessed and verified the requirements for the hat; I also discussed the connection between the name of the hat and the method for acquiring it.
The purpose of the consolidated summary answer, at least as I understood it, was to pin the requirements to the top so that users could quickly learn how to earn hats. Given that the event is over and nobody can earn any hats at all, that answer now collects the most useless part of each of the more detailed (non-joke) answers that came before it.
I'm not saying this sort of cleanup is never warranted or that moderators are not within their rights to do it, but I don't see the need for drastic measures here. What possible harm, or even inconvenience, would any user suffer who happens to find an old Q&A about an event that's no longer running, for which there exists a summary answer already pinned to the top by upvotes and acceptance?
I gather there were some flags on the question. There were answers that were clearly jokes. There were answers that were clearly duplicates. It would not have taken an inordinate amount of work to separate the wheat from the chaff, based purely on timestamps and vote counts. It did not need to be done right away; a lock would have been sufficient to keep the problem (assuming you grant it was a problem in the first place) from growing.
But the nail in the coffin of the "we did it for the quality of the site" argument to me is, if the whole point of deleting these answers was to clean up that Q&A and keep the useful bits, why are there still 80-odd comments hanging around on those posts? Those are the low-hanging fruit, after all; almost every one is idle chatter and/or obsolete. The notice that was added to the question even says it was "locked due to the high amount of off-topic comments generated."
I'm guessing this sounds like an angry answer but honestly, it's just that my sinuses are killing me and I have a tendency to extreme loquacity; to express how I really feel, here's Jean-Luc Picard. This was an unusually ham-handed reaction to a real molehill of a problem. It did a little harm in the form of lost reputation and bruised egos. It unjudiciously wiped a bunch of content, some of which was crap and some of which was not, from an interesting Q&A. The result is more concise, which is good, and more boring, which is bad.
Forgive me for not coming to a more satisfying conclusion, but my face feels like it's about to step into an alternate dimension. If I decide there's important content in my deleted answer that would be useful as part of the consolidated "FAQ style" answer, I'll edit it in. Anyone else whose answer was deleted is free to do the same. Okay. I'm going to Rite Aid now.