On Mastodon (and maybe also Akkoma/Pleroma/Misskey/firefish and so on) there is an option in the settings to auto delete your posts (formerly known as toots) with fine tune options if you for instance want to delete your posts but save your favorites and boosts. Several people have their toots older than one month automatically deleted. Before this was an option in Mastodon, people already did this with help of other software.
Lemmy is not very similar as StackExchange/SuperUser/Quora but in some threads Lemmy resembles a Q&A site so it makes sense to leave the conversations as is.
Regarding the most private social media question I’d think of Friendica, Hubzilla, and Pixelfed as best.