I was missing that one too and did some research about it a while ago. You can achieve the system-wide PiP mode in Safari with a bookmarklet. I posted the guide here: https://felixfoertsch.github.io/tip/2019/02/15/use-pip-via-b...
I've switched to Firefox on almost every other platform (apart from iOS). with Firefox 72, I've switch to Firefox only on MacOS as well.
[0]:https://apps.apple.com/us/app/pipifier-pip-for-nearly-every-...