I always used the “Unity Editor Download Assistant” to install a new Unity version over an existing one. The installer does show “Remove old installation” as far as I remember, but it does not seem to work in all cases.
I noticed in Windows 10 “Apps & features” list, that I have a lot of Unity entries there. It seems only alpha and beta versions add a new entry that does not get removed or replaced properly.
The reason for this is that when you install a new Unity version over an existing one, the replaced version will simply be deleted and the registry remains untouched. To remove the registry reference you have to use the uninstaller, which the installer doesn’t run when you install a version over an existing one, because Unity version folders are freely movable.
If you use the Hub then you won’t be able to overwrite an existing version (yet), but removing one takes just a couple of clicks and does run the uninstaller (removing the registry entries). So versions managed through the Hub won’t cause these problems.
Same here. Currently I don’t have any Unity-related software on my computer (uninstalled), except for these two. These cannot be uninstalled because physically they have been removed a long time ago.
I really think that the 2018.1.0b8 were (un)installed through Hub.