Does a Receive Only folder ever need to perform a full rescan (or enable the file system watcher)?
I mean perhaps this protects against accidental file deletion on the receive only folder side, but if I assume the risk that that will not happen on a backup-mirror target Syncthing node, I can safely turn it off, right?
The only purpose is to detect unintended modifications. If you don’t have any of those or you don’t care, you can turn it off (or turn up the interval very high at least, I don’t think there is an off switch).
Yeah, full rescans aren’t normally a problem if the hardware is decent, but if we’re talking about a low-end device with a slow HDD, then they may take a while even if there’re no changes. There’re also some quirks, e.g. specific to a concrete operating system like newer versions of Android where accessing folders with thousands of files can take forever.