Hello!
One thing I noticed is that the Animation package didn’t show up when I was using 2019.1, but when I switched to 2019.2 it appeared. I’m assuming that means this package is only available for 2019.2, which makes perfect sense. However it would still be helpful if these packages appeared in the list in the Package Manager, but appeared greyed out or something. This would allow us to see what we’re missing in newer versions of Unity! Often new packages appear without much fanfare, so being able to see them there would be amazing 
Also, I’m guessing certain versions of packages are also hooked to a certain version of Unity, so having those appear in the dropdown but greyed out would also be helpful
In short, it’s really helpful to be able to see all information, instead of having it entirely hidden depending on what version of Unity we’re using. Totally cool if we can’t install a package, as long as we know.
Thanks!
On a more serious note though, I think this is still a realistic idea, for instance if we showed these is optional, and clearly labeled, not simply greyed out. Clear communication is key!