As I'm not involved, so hopefully not breaching forum advertising rules, I suspect the answer Gareth can't provide is that one option would be to contract him and support the work to upgrade and test to 3.11, thus raising the priority of that work.
Another option would be to install it on a test 3.11 site and identify what the actual bugs/changes required are and contribute code changes for Gareth to review and pull into a 3.11.
There are lots of ways to assist and support an open source project like the Adaptable theme
If you do consider an alternative theme, you should always bear in mind that the same thing could happen to that theme at another core upgrade in future. The only themes guaranteed to be available on release of a core upgrade are the ones contained in core. Anything else (whether community or commercial) relies on the time commitments of the theme developer.