That is frustrating.
If you look at Docker Engine | endoflife.date (and click the "show more unmaintained releases button), you’ll see that Docker 18 support Ended 6 years ago (18 Aug 2018). It really isn’t too soon to upgrade.
So, yeah, if you’re that out of date on docker and your OS like you knew you were, then you shouldn’t upgrade Discourse either. 
I’m not sure whether it’s possible for Discourse to know what the underlying version of Docker it’s running under to be able to warn you on the UX.