Just to pitch in, I’m afraid I agree with Florian here, a custom parameter that I can check and uncheck is not something I would assume to be destructive, quite the opposite. Sure, I can hardly revert to the old metrics (from within the Glyphs file if I don’t use versioning), but I would expect kerning not to get lost.
I remember one case (not my design) where there were two exports, one synced (IIRC for layering) and one unsynced (to work as standalone).