http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.37.9...
SunOS 4.0 lists "dynamic linking" as new feature, released 1988
https://en.wikipedia.org/wiki/SunOS
AT&T System V Release 3 got shared libraries in 1986
https://en.wikipedia.org/wiki/System_V_Release_4#SVR3
While the first X11 release was in 1987, the fundamental architecture was designed already since 1984, and this architecture includes a heavyweight server that implements things that in most other window systems are done client-side.