I think that OpenIndiana is where those with general interest in Solaris on x86_64 should go.

https://en.wikipedia.org/wiki/OpenIndiana

SmartOS, for example, is a more specialized application of the scions of OpenSolaris.

Here is a list of other distros that originated from the Illumos efforts after OpenSolaris was terminated:

-DilOS, with Debian package manager (dpkg + apt) and virtualization support, available for x86-64 and SPARC.

-NexentaStor, distribution optimized for virtualization, storage area networks, network-attached storage, and iSCSI or Fibre Channel applications employing the ZFS file system.

-OmniOS Community Edition, takes a minimalist approach suitable for server use.

-OpenIndiana, a distribution that is a continuation and fork in the spirit of the OpenSolaris operating system.

-SmartOS, a distribution for cloud computing with Kernel-based Virtual Machine integration.

-Helios, a distribution powering the Oxide Computer Rack.

-Tribblix, retro style distribution with modern components, available for x86-64 and SPARC.

-v9os, a server-only, IPS-based minimal SPARC distribution.

-XStreamOS, a distribution for infrastructure, cloud, and web development.

https://en.wikipedia.org/wiki/Illumos#Distributions

Edit: From this blog entry, this is suspicious: "the committed support for Oracle Solaris until at least 2037" - does Solaris have a 2038 problem?

A few places, Fujitsu also has Solaris servers, and if you care about security, Solaris SPARC is the only production UNIX with hardware memory tagging in active use since 2015.

it's mostly OmniOS/SmartOS and other Illumos (descendant of OpenSolaris) distributions. All the Solaris 11 deployments i was aware of in mid-late 2010s are now either migrated to some sort of container setup of running on OmniOS.

My employer uses ZFS under AndrewFS (aka AFS) and I would bet dollars to donuts that the OS is Solaris.

Solaris proper, not Illumos?

I think Oxide Computer uses it.