It was 25 years ago today, on 15 September 1987, that Version 11 Release 1 of the X Window System (a.k.a. X11) was released. X11 has evolved a long way since then, but this 25-year-old technology out of MIT remains at the heart of every Linux desktop.
It only took three years to go from X1 to X11, but 25 years later, X11 remains in widespread use today not only on Linux but also other Unix-like systems such as BSD and Solaris. Ralph Swick of MIT announced the X Version 11 release on the behalf of other Massachusetts Institute of Technology developers, including Jim Gettys, Bob Scheifler, Todd Brunhoff, and the rest involved with Project Athena as well as many other independent organizations. Among the other vendors that contributed towards X11 were DEC, Sun Microsystems, and Tektronix. MIT Project Athena was the initiative by MIT, DEC, and IBM for creating a campus-wide distributed computing environment.
X11 was a major redesign compared to earlier X versions and marked "it's graduation from the research community into the product engineering and development community." Back then X11 was advertised for its forward-looking capabilities in terms of supporting deep frame-buffers, multiple color-maps, and various levels of hardware graphics assist. Back for the initial release 25 years ago, the hardware/software that X11 was known to work with included the "Digital VS-2, VS-2/RC, VS-2/GPX and VS-2000 under Ultrix 2.0, and 4.3BSD, most Sun Microsystems workstations with bw2 and cg2/3/4/5 displays under 3.2, 3.4 and 4.0, Apollo Computer workstations under SR9.5/6/7, and the IBM RT/PC with AED and APA16 under ACIS 4.3 (Not under AIX)." But if you happen to have such hardware around still after a quarter century, good luck getting the modern X11, via X.Org, to work on such hardware considering how much the reference X11 server has changed since then and the underlying operating system support for such hardware.
Among the features and capabilities introduced to X11 since its original release were support for the X Video extension, the X Font Server (although it doesn't serve much use today), XKB, Xinerama, XFixes, XDamage, EXA, KDrive integration, AIGLX, XCB, input hot-plugging, output hot-plugging via RandR, PCI domain support, new GLX extensions, Multi-Pointer X, DRI2, and a heck of a lot more. The code-base was also modularized. X11 in its modern form as of the latest code (X.Org Server 1.13) or the X.Org 7.7 Katamari is a vastly different beast compared to where it began twenty-five years ago, but it's still X11.
Back in the original days of X11, distributions could be ordered by sending a $150 USD check out to MIT in exchange for a single 9 track 1600BPI tape written in Unix tar format along with a copy of the printed X Window Systen documentation. These days, you will find X11 in the form of the X.Org Server in any Linux/Unix-like operating system. For those wanting the source code, it's a simple matter of cloning a Git repository (or several if wanting more than the X Server) and waiting a few minutes for the MIT-licensed code to download.
There's been some drafting of plans for X12, but no concerted efforts. X12 might never come given the tremendous progress and backing in recent years behind Wayland. Wayland with Weston (or other Wayland compositors) is looking to replace the conventional X.Org Server in years to come, but X11 will still play a role for many years in order to maintain backwards-compatibility with non-Wayland applications. For maintaining this compatibility there is XWayland, which is likely to be merged soon into the X.Org Server.
X11 isn't going to die anytime in the immediate future due to the sheer number of applications still depending upon this window system. Furthermore, non-Linux operating systems currently aren't targeted by Wayland (they're still struggling to port DRI2 and DRM/KMS support from upstream Linux) thus still making the X.Org Server very vital for the foreseeable future.