The folder confluence-home\plugins-temp seems to be used by at least the non bundled plugins to extract some libraries they depend on and which are packaged in the corresponding jar in META-INF/lib.
I have compared the content of this folder for a couple of plugins available from the plugin manager search menu or directly from the authors (calendar, cache, linking, multi-excerpt).
My understanding is that this is the standard way to add dependent jars to a confluence plugin, the plugin framework taking care of extracting them to confluence-home\plugins-temp when required.

The issue if that EACH time the server/confluence starts (or the plugin reinstalled I guess), it does it.
As an example on a DEV server I have <<616>> files in this folder corresponding in fact to the same 8 jars, 77 versions of each of them!!!
This server is not restarted everyday and this is a good thing or we would have 10, 20 or 30 times more files.

You need to do something.
This folder can be part of a backup and this is no good to waste disk space for nothing.

I simply suggest when confluence starts to clear the content of the folder.

Note: There is also a related bug for this situation that's been logged at CONFSERVER-52749