Speaking of solaris 2.5

Solaris 2.5.1 changed an environment variable export put out by the x library implementation
that provided a path to where various X resources were located.  Prior to 2.5, this
environment variable existed.  With 2.5.1, the variable was removed.

There were probably (other) portable ways to locate the resources, but Sun maintained that it
was not an interface change.  Basically, to them, ABI meant only the entry points.

So it all depends on how literally you want to treat things.  My experience is that they not
only break the interface, but they obtusely pretend they have not done so.  Others may have
had better experiences.  I haven't played with Solaris for a while.