It sounds like you’re getting affected by the updated support policy that Adobe announced a while back. Through the Creative Cloud app they now support installing only the current version plus one version back. Right now that means the 2021 and 2020 versions.
If you need an installer for anything older than that, you can check prodesigntools.com, specifically the Direct Download Links along the right sidebar.
Apple is doing something similar in that they provide software updates only for the current and previous two versions of macOS.
The big thing to keep in mind for both Adobe and Apple is that in the fall, both typically release major new upgrades. Because Apple and Adobe limit how far back support goes, every year another old version loses support. In the next couple of months:
- When Apple releases macOS Monterey this fall, support for macOS 10.14 Mojave will be dropped because the supported versions will be Monterey, macOS 11 Big Sur, and macOS 10.15 Catalina.
- If Adobe announces major upgrades at Adobe MAX in October like they usually do every year, then the supported versions will be those plus the 2021 versions.
2010 iMacs have dropped off the bottom edge of being supported by either Apple or Adobe because they can’t be upgraded past macOS 10.13. And that happened because when a new version of macOS is released, it usually doesn’t support Macs released more than approximately seven years or so before that release. But the iMacs are 11 years old.