Illustrator CC 2019 not loading Plugin

Supposedly releases are backwards compatible for a couple of versions, but it's sort of on you to make that happen. To do it, you'll need to know what suites you're using and and which versions are available in each release.

E.g. The AIArtSuite gets updated in most releases. That said, it didn't get updated in CC2019, so the version for CC2018 & CC2019 is the same - 20. But CC2017 was version 19. So when you're loading that suite (which I assume you use) you'd have to make sure you don't ask for a version that's not available (i.e. 20 in CC2017).

If you wrote your plugin for CC2017 then you're not making any calls to APIs that were added in CC2018 or CC2019, so you don't have that problem. It can get complicated if you're trying to leverage functionality in the higher versions, because you need to account for that in the lower ones. I don't think that will affect you though from the sounds of it.

You can get the version of the application from the AIRuntimeSuite, but of course, it also has the same constraints, so be careful.

Thinking about it, a cheap & easy way to do this would be to modify the code that acquires suites so that it doesn't fail when it can't get the requested suite, but instead tries with the next version down. So it would start with kAIArtSuiteVersion and if that fails, it would try kAIArtSuiteVersion - 1, etc. If you're using code Adobe ships in their examples for loading suites, it shouldn't be too hard to modify it to do that.