I don't know, we have a project that is still on 2.3.14 and Ruby 1.8.7, and we have not had any issues with compatibility. The old gem versions we have relied on all still work. And some of the gems we use are relatively new; for example, we use Dalli instead of the old memcache-client gem.
The only issue has been with our deployment environment — there is just too much effort required to support both 1.8.7 and 1.9.3 on the same box, so this particular app is still running on separate boxes that are configured using legacy Puppet scripts.
Well, the old gems will still work. But assuming you want to get any sort of security fixes, you need to backport all that yourself. I'm surprised you haven't run into more issues with gems targeting 1.9 now.
The only issue has been with our deployment environment — there is just too much effort required to support both 1.8.7 and 1.9.3 on the same box, so this particular app is still running on separate boxes that are configured using legacy Puppet scripts.