I'm generally limiting it to what the OS supports without third-party software, or in the "app" section whatever is officially supported by the developer (with a bit of an exception if the "official" app runs on another OS, e.g. the official Nextcloud app ported on FreeBSD).

This is because otherwise I'd have to keep track of all the software that's available by third parties (e.g. the Dropbox tool linked above - I'd have to check regularly whether it's still working or not).

Another example is file systems - you can probably read/write any file system on Windows or Linux if you get the right third-party drivers for it. It's very hard to keep track of this. New software is released, existing software stops working etc.

Of course the line is a bit blurred, e.g. the Webkit homepage says it runs on MacOS and Linux, so MorphOS (= Wayfarer) is not "officially supported". Yet it has been ported and runs and it's important to note that.