This.

If you're building a brand new, multi-language, multi-platform system that uses advanced open-api features - you will get bitten by lack of support in 3.1 versions of tooling for features that already existed and work fine right now in 3.0 tool versions. Especially if you're using a schema-first workflow (which you should be). For example, $ref's to files across windows/linux/macos across multiple different language tools - java, .net, typescript, etc.

If you need (or just want) maximum compatibility across tools, platforms and languages - open-api 3.1 is still not viable, and isn't looking like it will be anytime soon.