When a user posts, there should be an option to assign an expiration date so that the post and the entire thread gets auto-deleted at that designated time.

Meaning a post  should expire in 1 day, 1 week, 1 month, 1 year or after a user-defined period or never (and get archived).

A lot of posts become irrelevant, expired. I realize they scroll off the main page into page 99 but still.

Then there is the issue of security. Posting something semi-sensitive to get an opinion/brainstorm. Expire it in a day or two. So it never gets archived by google or by this forum. Maybe it's not so critical that it should never be posted but not desirable  to keep it around forever so that it doesn't get subpoenaed two years later in a civil court case. There are a lot of posts in that gray area. If a person posts his collection here (of whatever), which seems perfectly innocent at the time, then goes through a divorce two years later and the ex-spouse finds out about his membership here and prints all his posts from archives from the forum to show in court he has to sell it / split it.

Perfectly reasonable things posted today may not be reasonable 10 years from now.

Then there is the matter of web site resources.  Why keep irrelevant junk around. The web site would load faster from the user perspective.

Message auto-delete is a standard feature in all major messengers such as telegram, signal, whatsapp, viber, et cetera.  They are not really just messengers but complete ecosystems since they too support chats and channels.  We are not in 2004 anymore.