Configuration option for global post lifetime
I believe an option for instance administrators to be able to set default post lifetime would be a good idea. If one's instance became "popular" (maybe thousands or tens of thousands of users), I'm not sure how one would manage the infinite storage requirements required if posts stuck around forever (and didn't have the financial resources for such a thing).
I could see things like pinned posts would probably want to be an exception, but beyond that, I'm not sure what data really should stick around forever.
There are also users who would prefer to not have their chats/s**tposts available forever (and that would be a nice additional feature as well, to let users adjust their own maximum post lifetime).
As there is a remote_post_retention_days: setting, I would hope that a complimentary local_post_retention_days: setting would fit the bill.