A user should have the option while posting, to designate the expiration time interval.
i.e. this post (thread) will self-delete in 1 day,  1 week, 1 month.. 1 year. or a custom time.

Why?

Lots of reasons.

1) Avoid leaving self-incriminating info about yourself in case someone later decides to scan your social media stuff.
Things you say today may sound innocent but a couple of years down the road someone will profile you for a job, or a court case comes up (Like divorce, with the ex looking to find as much dirt on you as possible)

2) Do not clutter the community with irrelevant junk info, something which has no value beyond 3 days.  Say, "I am visiting DC this week, what to see".

3) Consider all the major messengers/social media like telegram, whatsapp, signal, et cetera have the self-delete feature built-in.  Except facebook but we know it's retarded.

I realize its not up to the admins, the software vendor would have to add that feature. Or maybe upgrade the entire board to something which already has this.  Right now the way it is, don't say anything here you don't want to see on the front page of Washington Post. (Not that this will ever change).

Sounds like a good idea

just don't say things you wouldn't say in public.