The latest version of my WordPress theme has a bug. The version before it does not have this bug. Is it okay for me to stay on the previous version long-term? I don’t use many plugins, mainly HTML and custom CSS which are written for my child theme. Is it possible for me to make a particular version of my WordPress theme future proof? Specifically, compatible with future WordPress versions?
I'd say it's a security risk that will only increase with time. How do you see making it "future proof"? It's like asking a car manufacturer for a car that's accident-proof - they don't make these. That's what car insurance is for. The WordPress version you're on is only good for having protections against already-discovered vulnerabilities. New ones keep appearing, so patching them is part of the reason why new versions keep coming out.
So, to extend the analogy of automobiles, your "car insurance" would be to pick a reliable host with good support, keep your WordPress up-to-date, and follow best-practice advice for protecting WordPress site against attacks (lots of it out there, just do a simple web search) while balancing it against your particular situation (because not all security advice can always realistically be followed in all cases).