One thing that was recommended to me by someone a while ago, is that, unless you need it for something specific, mount your media in Plex as read only.

Plex has functions where you can delete content from the library from their UI. If you need that for some reason, obviously don't make it read only. If you're hoarding the data, and therefore never delete it, or use an external system for deleting files, then RO all the way.

The only caveat to this is if you're using a local disk on the Plex system, which then shares out the drive/folder for adding new content, in which case, you're screwed. It has to be rw so the OS can add/remove data.

In my case, as I think may be common (or at least, not rare), my back end data for Plex Media is on a NAS, so it's easy to simply have the system running Plex, mount that network share as RO, and you're done. The data on the NAS can be accessed and managed by other systems RW, direct to the NAS.

Since Plex is exposed to the internet, if anyone with sufficient rights is compromised, in theory, an attacker could delete the entire contents of your media folder with it. If you limit RW access to internal systems only, then that risk can be effectively mitigated.