I was running MacOS El Capitan (10.11) and wasn't able to update my old versions of Photoshop and Illustrator because of OS compatibility. Now that I've upgraded to Sierra (10.12... yes I know I'm a little slow in updating my OSes!), the Creative Suite tool is telling me the only available updates are now the actual most recent updates... which do not work on Sierra.
I know for fact, for instance, that Photoshop 2018 and 2019 work on Sierra. But I'm stuck on 2017. And for my Illustrator 2018 v22, I know there is at least one more update, to version 23, which I should be able to download. But it won't give me any of those choices. Instead it says only Photoshop v22 and Illustrator v25 can be downloaded... and I don't meet those requirements.
This is not how the Creative Cloud tool used to work... I feel like it always figured out which version you had and gave you the maximum path to download. But not anymore??
I think I may have a similar issue with Lightroom.
I'm on a 2013 iMac i7 which I'm pretty sure has way more power than my 2011 MacBook Pro which is happily running Photoshop 2018 and Illustrator 2018 verison 23 ... so why is Creative Suite preventing me from updating on my iMac?
Adobe radically changed their policy on old versions to be offered. It is now current + previous one, that's all. There is simply no way to go back further. Your upgrade from one obsolete system to another one hasn't made any difference. Each app has its own system requirements you need to check separately.
For Photoshop 2021: macOS 10.14 (Mojave) or later
For Photoshop 2020: macOS version 10.13 (High Sierra), macOS version 10.14 (Mojave), macOS version 10.15 (Catalina)
(macOS 10.13.6 or later is recommended for best performance)
If you'd done this a month ago, Photoshop 2019 would have still been available: macOS version 10.12 (Sierra), macOS version 10.13 (High Sierra), or macOS version 10.14 (Mojave)