Not exactly the best solution, but when I wanna download something from YouTube I use JDownloader2 which is running using Java and it's cross platform. It's still updated regularly and it still runs on the latest XP supported version of Java just fine. This is not an accident, but rather a nice coincidence given that JDownloader2 is cross platform and therefore supports all OS capable of running Java, including a wide variety of Linux distro which are running a very old version of Java, which is why it still works perfectly fine on XP at every update. As for the FFMpeg libraries it uses, I'm of course using Reino's fork that he regularly publishes on Doom9 and that have been already mentioned in this thread.
As for things I wanna watch on my PC without downloading them, with M108 (the chromium fork), all videos work, including VP9 ones and AV1 ones as they're royalty free codecs whose decoders are included/embedded in chromium however they all run via software decoding due to the nature of the XP compatible versions of chromium. Nonetheless, for anything up to 1920x1080 60p software decoding on a quad core is fine. I don't of course expect it to be playing 4K or 8K footages via software decoding on a puny 4 cores CPU, but realistically most of us probably have a FULL HD 60Hz display anyway, so we're gonna be more than fine for the time being.