Custom folder types can be created in the registry. That's the easy part. The difficult part is applying a custom folder type to an individual folder or an entire folder tree. There's no way to add a custom folder type to the list of 5 types that are available under "Customize". Currently, it would require a complex custom Explorer extension to achieve the desired result. It's something I may try to program some day (if Microsoft doesn't build in such a feature), but I'm making no promises, as my investigations so far have shown that it's a significant challenge.
Microsoft has not made any announcements in regards to adding any custom folder type capability and I wouldn't hold my breath. The Windows 11 File Explorer has actually regressed, in regards to folder type features, from Windows 10. IMO, Microsoft should focus on those bug fixes first, but their priorities are a mystery to me. Explorer also needs some other basic features like a simple checkbox to turn off automatic folder type discovery, an option to manually "rediscover" the folder type for a single folder or an entire tree, improved sort ordering (e.g. option to ignore extensions when sorting), etc. And, not least of all, proper support for long paths (i.e. >260 Characters).
Some third party file managers, such asDirectory Opus, do many things better than Explorer, so that's an option to consider in the meantime.