>>/19055/
> Is there an expiration date for the uploads? 
No.
> files pruned from the site after some time?
Orphans are pruned.
If all the posts with the same media are deleted, that creates an "orphan".
> How much is that time? 
Within weeks. Kinda random.

 >>/19056/
 >>/19057/
There was a botched migration in 2018 or so which resulted in database corruption. There is a thread about it here on operate somewhere. The result were bunch of lost media, broken files, some just left without thumbnails.
Well, that's about it.