Some time ago there was dismay on Mastodon (and Pleroma ?) instances about data scraping. Before that happened there were privacy concerned people automatically deleting their own toots, limiting them for example to the latest toots for thirty days. With Lemmy things are different since Lemmy is a link aggregator rather than micro blogging, and Mastodon has boost option (Called replay with Pleroma ?). However deleting your older comments (but not posts) automatically would be really nice for privacy reasons.
See also my other new comment about issues
Posting a URL or an image can show a little bit about yourself, but comments and a posted question will show a lot more. Leaving the old posts like they are makes some sense since people can benefit from reading it without a very high chance of giving up your own privacy.
Reading also your other comments, I don’t think I agree. Posting stuff publicly is well… public. If you don’t want stuff taken out of context or otherwise abused it is probably better to not post it publicly in the first place.
Note however that legally speaking the barrier for “private” posts are pretty low in most countries, i.e. posting something on a invite only (even if the invite link is public) Discord channel is apparently considered private communication and thus you can press charges against someone publicizing it without your consent. I think that is why some Mastodon instances do not have a public feed, but with federation it becomes a bit murky.
But IMHO maybe what you are really asking for is invite-only Lemmy communities?
Maybe not that bad idea: password protected communities 🤔