What I meant by that is, the software versions in Debian 9 were frozen in 2014 when it was first released. Your software is now 6 years old. It continued to receive bug fixes and security patches, but it didn't get any newer in all this time.
And yes, it's a shitty thing that so much software always expects all of us to have the latest and greatest the instant it's released. My pet peeve is Python apps starting to require version 3.8 as a minimum literally hours after it hit the official download mirrors. That sucks.
Unfortunately I don't have specific experience with Debian 9, but most Unity games wouldn't run for me even on a more up-to-date Linux. Some at least had the decency to say it was due to expecting a GPU with OpenGL 3.0 features; most would just crash. Sadly, I doubt this can be fixed easily or at all. Sorry.