Re: Took them long enough...
Ignoring X11 for a moment
Technically, X11 is a protocol. The API is Xlib. Ah, sweet pedantry.
X11 is a relative newcomer - only been around since '87. Various UNIX APIs (later standardized by POSIX and X.Open, and then the SUS) and parts of the C library (later standardized by ANSI X3.159 and ISO 9899) have gone unchanged since the early '70s.1
Similarly, MVS goes back to '74 and many of its APIs live on in zOS today.
In Microsoft's defense, sort of, I will note that there are still available Windows APIs that haven't changed since Windows 2.0, such as SendMessage. (They may date back to Windows 1.0; I never wrote code for anything prior to 2.0.) So that makes those APIs as old as version-11 Xlib. Of course, keeping some APIs says nothing about the ones that were discarded.
For that matter, NTVDM maintains various DOS APIs, such as they are, and it's still present in 32-bit Windows 7. That pushes the oldest APIs Microsoft still supports back to 1981, though purely as a curiosity.
1Roughly 1973; before that the C language was sufficiently different that the APIs can't in fairness be considered the same in most cases.