I ended up finding that information after I posted that. It would be infinitely more useful if they posted in the description of the “cannot be updated” information about why exactly instead of having us fish around the Internet for the it.
Guess it’s time to try and sideload Catalina onto my 2010 Mac Pro.
It happened “suddenly” because as Adobe does every year now, on October 26 they announced the next year’s round of Creative Cloud upgrades. Every year when the new versions come out, the system requirements ratchet up a notch.
This year, your version of macOS became too old to be included in the list of supported versions, so the latest updates are not available to be installed. Adobe follows Apple support guidelines: Both Apple and Adobe provide software updates for the last three versions. The last three versions are macOS 10.15 Catalina, macOS 11 Big Sur, and mac OS 12 Monterey.
What you can do is use a more recent version of macOS. If your Mac Pro cannot be upgraded to macOS 10.15 Catalina or later, then your choices are to stay on older Creative Cloud software, or buy a newer computer.
Adobe provides only the last two major versions of applications in the Creative Cloud installer, so even if you are able to upgrade your Mac to macOS 10.15 Catalina, the Adobe applications that still support that will probably disappear from the Creative Cloud installer next fall. So be prepared for that, whether it means trying to upgrade macOS further or geting a new computer. By next fall a 2012 Mac will be a decade old.