IBM did give up on marketing OS/2 by the late 1990s (and the PC division arguably did so around the time Windows 95 came out), but I am not aware of IBM explicitly telling new and existing customers that Windows is the way forward for existing systems.

A slightly different way of answering your question is that sometime after the PS/2's launch, IBM began explicitly noting that its software and peripherals were compatible with non-IBM computers. PC DOS 5.0 might have been the first. <https://np.reddit.com/r/vintagecomputing/comments/1jleun5/ni...>