Yes. There were a number of rapidly developed versions up to X10 in 1986, which was the first one released under a free license (and thus the first one widely adopted). X11 was an overhaul of X10 intended to increase portability/compatibility, and also the first version developed as a community open source project rather than internally to MIT. That came out in 1987.

As for why X11 was the last major version: the protocol has remained backwards compatible since then. Clients created decades ago can still operate with modern X servers. It's pretty cool!

http://www.theresistornetwork.com/2013/12/a-testament-to-x11...

It happened really fast. X1 happened in may 1984, x6 in 1985

See https://en.wikipedia.org/wiki/X_Window_System#Origin_and_ear...

Yes. 11 is the version of the protocol, which didn’t change fundamentally after X11, so they moved to X11R2, R3, etc., and adding extensions.

11 is the protocol version. With multiple implementations.

I think once extensions were introduced, they probably didn't need to extend the protocol.

Similar to HTML: a lot of early evolution in the early releases. Then a long wait for version 5. Version 4 had a lot of neat stuff like math typesetting. Html balkanized during the commercial browser wars. Tim made a valiant effort to hold it together these past decades.