I haven't seen any UNIX systems in operation for a few years. In a previous job we had a lot of AIX and HP-UX systems which I had to manage, and even a Tru64 system! I would assume the AIX and HP-UX systems are still in use.

I always keep an eye out for UNIX jobs but I find that a lot of {companies,recruiters} use the term Unix when they actually mean Linux, which bugs me a little.

Every company I have worked for has used Red Hat Enterprise Linux for most of their systems. Many would also use CentOS for non-critical systems and I have seen a few Debian systems. Usually Debian was installed on old, long running systems that some greybeard put there years ago and nobody wants to disturb "because it just works".

Having a Red Hat support contract does have its uses. One company brought Red Hat in to assist building an Openshift environment and occassionally we have sent crash dumps for analysis. Most of the use I get from it is access to their knowledgebase.