SPARC has become a second class citizen on Illumos ever since the demise of OpenSXCE (2013-2014). The leading commercial Illumos vendors Joyent, Nexenta and, up until 2018, Delphix, seemingly didn't want to invest into SPARC, being it tightly linked to Oracle; supporting old SPARC hardware probably just appeared nonsense. That said, SPARC support on Illumos for legacy Sun hardware is kept alive by few hobbyist projects:
Tribblix (my illumos distribution of choice, really worth a try if you like either NetBSD or Slackware),
v9os and
DilOS