YouTube Music still has this feature and it even uses the die icon, just like always.
> you could upload your library to it.
You can also do this in YouTube Music.
> But they killed it
They rebranded the exact same code base with largely the same staff. I don't know why you perceive this to have been a break.
This is actually completely false. (Source: I worked at Google at the time this transition was made, though not on GPM or YTM).
Two completely different codebases and teams and in fact YTM launched missing a boatload of features that Play Music had. Including annoyingly the ability to use our own Chromecast features on the desktop/browser version. (I worked on the Cast team). There were quite a few angry internal buganizer tickets.
It was the betrayal that turned me off Google.