Ladies and gentlemen this is truly a strange one. Recently Safari.app for macOS stopped streaming video from any site including YouTube.
Immediately I looked at the usual suspects. Browser cache, extensions, nvram, OS integrity. None could explain it but curiously everything worked in private browsing.
At the same time I realized I had run out of iCloud Storage. Remediating this issue resolved the Safari issue. No idea why but that’s technology, baby.