I know that X11 is called that because it's the 11th version, but if it reached that version back in the '80s, then why hasn't a new version been released since then? We must be running ridiculously old software :).
closed as primarily opinion-based by Thomas Dickey, llua, Christopher, Jeff Schaller♦, grochmal Dec 30 '16 at 3:09
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.
I know that X11 is called that because it's the 11th version
The network protocol is the 11th version. For example, I'm running X.Org X Server 1.18.4, but it still implements the X11 protocol (along with optional extensions that didn't exist when X11 started).
if it reached that version back in the '80s, then why hasn't a new version been released since then?
The basic protocol hasn't needed to be changed, so nobody changed it. The X community followed the sound engineering mantra "If it ain't busted, don't fix it." That being said, a lot of extensions have been heaped on the basic X11 protocol since the 80's.
It's like you can buy a new car (with a modern flex fuel engine) but you still accelerate by pressing the gas pedal steer by turning the steering wheel, just like in the 80's (the same old protocol that didn't need changes).
Now, if you want a car with a joystick instead, check out Wayland https://wayland.freedesktop.org/ ;)
It's called X11 because that's the name of the protocol: how the clients and servers talk to each other.
The full name of the current X protocol, however, is "X11R7.8" (see Wikipedia's release history), and it's very different in capabilities from the X11 that was released in 1987. Much of the X11 functionality in that original version is no longer used, replaced by modern extensions, and is retained solely for backwards compatibility.