Most customers were on Ubuntu/RHEL/Windows. There was very little on FreeBSD, AIX or Solaris. We had zero interest for HP-UX, I think that is dead enough to be ignoreable. Banks and financial services have a tendency towards AIX, while Solaris I think was primarily one customer that had a lot of legacy. AIX and Windows were the biggest pain in the ass, but every time we tried to kill support for it, people discovered sizable contracts that had been signed with us (yeah, our tracking in salesforce was bad).
My background is that I learned C and Unix on a Tandy 6000 back in 1989/1990, then in college used and worked on a wide variety of O/Sen (Dynix, BSD4.3, Digital Unix 4.0, SunOS 4.1.4, Solaris 2.4+, Irix 5.x/6.x (i think), something that ran on a VAX, NetBSD and later Linux). I ported NMAP to a bunch of those and did the original GNU autoconf work on it. I've been mostly Linux since 2001 (Amazon from 2001-2006).