HunchBluntley: For some odd reason,
Co-optimus doesn't seem to have a way to filter computer games by operating system, only to filter PC games from everything else. However, unsurprisingly, they DO have a filter specifically for online co-op, and given the site's focus, I would expect it to be much more accurate than such filters on any store. So, you could filter for that, then cross-check any results that check whatever other boxes you care about on your storefront/search engine of choice to see if those titles have non-Windows versions. Still cumbersome, unfortunately.
You should also consider asking in a fairly focused subreddit such as r/linux_gaming and/or r/macgaming.
yeah I guess I'll have to go down the reddit route after all
online and multiplayer should be two separate filters in every store tbh, as they are always misleading... be that coop or competitive or untagged completely, at least that separation should be made more obvious