I wonder where Solaris is still actively being deployed and used.

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?

No 2037 problem. That's just 25 years after they killed the product.

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.

AmpereOne M with MTE is out nowadays

Good to know, still I bet there are more Solaris SPARC deployments on the wild than GNU/Linux on AmpereOne M.

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?

Either way, SPARC and the entire family seem to be entirely dead in the grand scheme of things. I don't know why anyone would develop for this platform.

Solaris and Illumos are available on x86

But illumos doesn't run on sparc... granted I don't have the hardware, but if I did, it would be nice if I could use illumos.

Tribblix does, which is based on illumos - I have a V210 I installed it on, not too long ago...

Nope.

Oracle SPARC S7, T8, and Fujitsu SPARC M12 still supported

supported sure but its an ancient dying platform. sparc was discontinued almost 10 years ago.

I think Oxide Computer uses it.