activitypub server that "freezes" posts made locally after a configurable interval (say, six months by default, or some such), removing them from the database and making the posts non-interactive, but rendering the frozen posts in html and json as flat files on disk, with a small script to check for authorization on a get request that passes the flat file through if authorization is successful, but not recording new favs, replies, boosts, etc, and returning a well-specified error response whenever a remote actor attempts to do so.

Pretty easy. Set an expiration date on commenting. After that if somebody comments, return 403. Available today in a number of fediverse platforms that you've obviously never used.