A general description of the proposed change and reasoning behind it is on GitHub: https://github.com/LemmyNet/lemmy/issues/3697
Linear execution of these massive changes to votes/comments/posts with concurrency awareness. Also adds a layer of social awareness, the impact on a community when a bunch of content is black-holed.
An entire site federation delete / dead server - also would fall under this umbrella of mass data change with a potential for new content ownership/etc.
AccountDelete has a marketed feature to overwrite all post and comment content.
Hmm ok, false sense of security there since another advertised feature is the open API (meaning no restrictions on scraping bots so there will definitely be archives of deleted posts), but whatever.
How does this sound: encrypt the comments in the db using a random key stored in the account row. Then at account deletion, overwrite that key, so the comments can no longer be decrypted. Maybe there is a way to purge those comments altogether during the next VACUUM. No idea how often that happens though.