I read that the illumos Foundation is no longer

There's lots that can be ported to illumos from BSD's such as LLVM, NetBSD's make utilities, NetBSD's video drivers, FreeBSD's wifi drivers. FreeBSD used to have the best graphics drivers, and NetBSD was far behind, but NetBSD surpassed FreeBSD in video drivers, and no one realizes that.

the illumos foundation never really did much, so it's hard to notice that it's gone

It was gone as of last year. They did hold the core together. BSD's have less common organization and more organizations and communities holding them together.

and yes, tons of possibilities, if people have interest and time to do that

Everything needs a backing foundation though, even if it's not illumos

I don't think the foundation did anything to "hold the core together" - the development community did that on it's own

I recently learned that SunOS was a BSD. When it became Solaris, it started having SysV.

many open source projects do fine without a backing foundation

That's good

yep, early SunOS was forked from BSD 4.2, SunOS 4 (aka Solaris 1) started merging in some SysV components, and SunOS 5 (Solaris 2) was rebased on System V Release 4 (SVR4)

There were no books on illumos

and was not well received in some quarters.

it took *years* for Solaris 2 to get anywhere near as stable as SunOS 4.

but I'm not bitter about it. Nope. Not at all.

[illumos-gate] 17536 overflow checks ruined by int promotion -- Patrick Mooney <pmooney⊙pc>

Some history: computerparkitecture.substack.com/p/the-long-mode-chronicles

(I commented there's no mention of S10's AMD64 support with its FCS.)