Surely that's up to the individual. It maybe shouldn't be the default setting, but some people might have very good reasons to not want a permanent record of every silly remark they've ever made, sitting on the internet for all eternity.
On my Mastodon account I've set it up to auto-delete posts after 4 months because I believe social media should essentially be ephemeral.
Exactly. It's like a conversation. I don't want or expect it to stay around forever. A few things that I do want to be around, I can publish in a blog.
I take auto-delete as being a huge feature. Some platforms have it, some don't, and it's not really advertised, but it should be.