Hello,
As a temporary solution, I threw together a small app in Automator/Applescript which basically loads Youtube in a new window with iPad as the user agent (long story short: Youtube behaves like it would on a landscape oriented iPad, with resizable window). I also made a service so it is possible to right click on any Youtube URL in Safari and "Watch on Youtube Max" (dull name, I have no creativity left), and of course the standalone app works as well.
1. Run installer (provide password when asked, due to installing services).
2. Run app from your Applications folder or
3. Use the Services context menu when selecting a Youtube URL in Safari.
I was half asleep when I made this, and I have not had the chance to test it any further, use on own risk (although it should be perfectly safe - after all it only opens Safari browser windows, which is already installed on any Mac).
I hope this will be resolved soon anyway 🙂
Cheers,
Andreas
Download:
https://goo.gl/GHAQCO