It looks more that broadcom did not (want to) pay youtube for vp9 licenses... so they had to stop vp9 support.
The solution is watch youtube via your smart TV, AppleTV or Google Chrome device. This will 'always' work without hacking and ton times per year adapting youtube plugin when youtube make changes to their security stuff...
Doh, I naively thought it was royalty-free (thus preferred to say H.265 / HEVC)
https://www.webmproject.org/license/
https://www.webmproj.../faq/#licensing
Note the enigma2 youtube plugin does display videos of Youtube...simple enough.
Or you can install kodilite on OpenPLi and use the addons.
Anyway, for the initial poster info, iirc last drivers working with Youtube-App where at the 6.0-rc epoch.
Somehow, very seldom, the Youtube-App videos do play...I can't say why...normally you only see one frame on the fb when exiting the app.
Cheers
A.A.