In a very old PMS changelog it says: FIX: (API) Allow PUT /library/metadata/X to take multiple IDs.
I’ve tried numerous ways to achieve that: /library/metadata/X,Y/refresh, /library/metadata/X/Y/refresh, /library/metadata/X;Y/refresh, /library/metadata/X&Y/refresh, /library/metadata/X+Y/refresh, to no avail.
Hard to believe that no one in Plex can answer these kind of questions… even harder if you consider this should be already documented in a proper official and updated documentation…
PUT /library/metadata/X does accept multiple comma separated values like the changelog says. You can test this by looking at the PMS log. PUT /library/metadata/X/refresh does not accept csv.