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.