Hacker News Reader

You could do this on pretty much any site, given access to javascript. I remember doing this for facebook before I deleted my account ~6 years ago.

There are people on mastodon/pleroma who have scripts that automatically do that for posts older than 30 days, etc.

Snapchat and instagram stories are ephemeral, but those are different from traditional social media with posts.

Yes, snapchat and instagram stories are examples of temporary social media products

Most of the chan boards do this.

It's also worth noting that many of the chan boards are also publicly archived, though the effect of that is mitigated by no/ephemeral identities.