They're working on updating 3 so that doesn't happen, but they've dropped support for 2 so they aren't looking at fixing that anymore. But they know that many people still use version 2, so they're just stating up front that if you plan to continue using QLab 2, then you can't update your OS. Or if you update your OS you will need to update your QLab (and also rebuild all your show files from 2 to 3).
At this point the only uses of Qlab 2 should be existing shows that are already installed. The "best practice" for an installed system or show control setup is that once the show is open or an installation is complete, functional, and verified is that software on show control machines, operating systems, firmware on devices, etc. should only be updated if there is an
extremely good reason. As soon as you update an OS or firmware, you potentially introduce unknown factors into the show environment.
An ideal QLab setup for most installations would consist of two computers (we usually specify mac mini, but other macs will work too). One primary machine, and one live backup that runs in parallel with the primary machine. Once the show is open... don't touch