Can't install two versions of Photoshop as of May 2019?

Before the Adobe policy change limiting  legacy CC version access to this current one and the major release behind it ,  I had older Photoshop versions installed on my computer.  I am on an individual plan so those older versions are still installed on my system and I can still launch them from my computer's start menu.  The only difference is that they are not showing up in my CC Desktop application.

If you're on an Enterprise plan, your administrator has in all likelihood taken Adobe's recommendation and uninstalled older/unauthorized versions for legal reasons.  I suggest you talk to your administrator.

There may have been recent changes in the help pages in connection with previous versions now only being the last three major updates.

Photoshop has had quite a few minor updates, and all of those were tweaks to the same version. If you need to install the previous major version of Photoshop, you would need to take the 19.x one, and not one of the 20.x ones. Same with Illustrator, you would have to go back to 22.x and not take one of the 23.x one.

You could install the 19.1.8 version of Photoshop, and although reporting that there is a problem in 20.0.4 that wasn't in 19.1.8 isn't as useful as saying it was ok in 20.0.3, it would still get looked into.