In the 90s, perhaps not massively, but gaining ground very early in the 00s. I started my career in 2000 and most of the credit-card related stuff I built until ‘05 was targeted at Windows, Linux and Solaris, with a variety of other Unix platforms depending on the client/project.

But the x86 I was referring to in my comment above, Stratus, was (maybe still is?) an exotic attempt to enter the mainframe-reliability space with windows. IIRC it effectively ran two redundant x86 machines in lockstep, keeping them in sync somehow, so that if hardware on one died the other could continue. I have no idea how big their market was, but I know of at least one acquirer/issuer credit card system that ran on that hardware around 2002-3.