Yep, but does it really important?
It's pretty clear why custom protocol is not supported, because it wouldn't be possible to validate your link, you can claim that some web domain belongs to you, and you validate it using a special file on this domain, but what about your custom protocol? Hi how would it be possible for the system proof that it's your domain on this protocol, imagine someone would claim that
onion://google.com belongs to you, how would system validate it? There is no way to support all possible protocols
What kind use case you have for it exactly? Or it's just theoretical