@guenther @brynet we've shipped O_CLOFORK support since Solaris 11.3. I'm not sure how it handles the case of CLOFORK without CLOEXEC though. @jlevon did the original implementation back in 2014, and Casper Dik updated it in 2020 once the standard committee decided to adopt it.
I believe AIX & MacOS had implemented before we did, as had a proposed patch for Linux that wasn't accepted at the time.