TLS and HTTP are different things. TLS is being used without HTTP in lots of cases.

Besides, even if such extension had existed it would've been easy to write X in TLS header and Y in the HTTP payload to circumvent the ban, like the domain fronting[1] trick currently being used by e.g. Signal.

[1] https://en.wikipedia.org/wiki/Domain_fronting

That would be such a bad idea, lots of websites send data that should be secret on the query parameters.

Services like google share IPs amongs their services. If SNI was encrypted, youtube.com could not be blocked unless the entire IP space of google is blocked (which would be very hard to do since nearly everyone relies on gmail).

Kazakhstan blocked major part of gmail functionality at one point of time. You couldn't download attachments, images didn't work, may be something else. I think they tried to block blogspot. but shared IP broke gmail. It was broken for months, nobody really cared except people pointlessly ranting on forums. Those, who needed working email, migrated to other providers or used proxy. I'd like to live in the world without censorship but I don't see this happening, so I'd prefer to minimize censorship damage at least.