There are still a lot of IBM mainframes out there. Far less than there used to be, but IBM mainframes remain (even in 2020) a sector with many billions of dollars of annual revenue (if you add up hardware and software and services across both IBM and the ISV ecosystem). And the vast majority of those mainframes run z/OS. (They also run other operating systems – z/Linux, z/VM, z/VSE and z/TPF – but many of the sites running one of those other operating systems run z/OS as well.)

The UNIX component of z/OS is not merely a compatibility layer, a lot of operating system components and applications now rely on it. For example, the REST-based z/OS management console Zowe that IBM is now promoting runs under z/OS UNIX. My understanding is that IBM prefers (where possible) to develop new OS components and applications to run under z/OS UNIX rather than the traditional z/OS APIs. Anything written in Java (or other newer languages, such as node.js) is running on z/OS UNIX. So even if a z/OS site isn't doing anything directly on UNIX, they are almost certainly using OS components and applications which directly rely on it to function.

Obviously z/OS is absolutely dwarfed by Linux and macOS, but it is probably the healthiest of all the commercial Unixes – Oracle has put Solaris in maintenance mode, same for HPE and HP/UX, HPE killed Digital Unix years ago; AIX is healthier than Solaris or HP/UX, but I think z/OS is even healthier than AIX. z/OS licensing fees are a lot higher than AIX licensing fees, and the non-UNIX parts of z/OS (JCL, VSAM, CICS, IMS, etc) make it far harder to migrate off than AIX is.