It depends on a particular community.
Communities like nostupidquestions are not expected to have posts with important news or problems one might google a year later.
Tech-focused communities, on the contrary, are expected to keep posts forever, so that important information will be accessible. However, some people delete their posts after their problem is solved. This is highely discouraged.
And Lemmy ain’t just one site. Yeah.
BassTurd@lemmy.world 4 weeks ago
Some instances are hosted and paid for by people that don’t have lots of money. Purging old posts to save costs is a valid reason to do so. Whether or not that’s the case, I have no idea.