This morning there was a notice of an update to my Mac Pro 5,1, running Mojave (Mac OS 10.14.6). I clicked through to the Mac App store, and was told Sonoma was available.
I clicked through, and lo and behold, under “Compatibility”, it says
“Works On This Mac Pro”.
I doubt very much this is true, nor would I make such an upgrade without figuring out which software would still work yadda yadda, but thought it amusing.
Here’s a screenshot for your amusement:
Also running Mojave on a MacPro5,1 – but I don’t see that update notice. At least, not yet…
Just for those who perhaps don’t know, the Mac Pro 5,1 is the cheese grater model that debuted all the way back in 2010. 
IMO, one of the best machines Apple ever released - a reasonably priced LAN server Mac, designed for ease of upgradability. Granted I’ve upgraded 2 of its 4 internal drives to SSD, added lots of RAM, and upgraded its video card, but it still chugs along /knock on woodish desk. About the only thing wrong with it currently is one of its 2 DVD/CD burners failed, but I only need one these days anyway.
I believe it is possible to upgrade it to a newer OS than 10.14, but I’ve read that Photoshop CS6 will no longer run, so I haven’t pursued that option.
I agree, I love my Mac Pro, and I hope it lives forever. There is no point in doing the modification to go beyond Mojave if you want to use Photoshop CS6. I’m in the same situation, and in fact have not bothered to take that drive beyond High Sierra.
Yep. And According to Apple, Sonoma is definitely not compatible with it. No Mac Pros older than the 2019 model are listed as compatible.
Sounds like a (potentially dangerous) glitch in the App Store listing.
Still listed as available for me today, and I’m sorely tempted to click the “get” button just to see what occurs (assuming I could force-quit before anything actually started to happen). Will the Mojave App Store really download the Sonoma installer?
It certainly won’t install even if it downloads, so I’d click it. 
Some developers provide older versions of their products to the App Store, so when you try to get the latest versioin you are told it’s not compatible with your current OS and will offer to give you an older version. I noticed that it’s not very smart in determining whether you already have the most recent version for your OS and will then just reinstall the same version you already have.