@tshirtman There are usually scheduled tootctl scripts that admins run to clean up stuff older than a certain threshold. But the cache folder itself can't simply be wiped without basically everything on the instance breaking (including emojis and avatars), and it's often very painful to fix it.
It seems like one of those bad design decisions where the cache folder has been used as a dropbox for almost everything, including things that the software is supposed to store long-term.
So even with the cronjobs scheduled to run every day and very little local traffic, an instance connected to a few big relays ends up having many GBs of media cache that it can't touch.