Work continued on trying to prevent bugs that result in killing all processes on the system by mistake, in response to customers hitting https://github.com/sudo-project/sudo/issues/458 last year, and bugs in other programs using -1 as a pid value in kill() calls by mistake.
Previously, in #OracleSolaris 11.4.83, killpg() was made to reject the always invalid process group id of 1, instead of turning it into the "kill all processes" argument of -1 to kill().
Now in 11.4.90, the kill system call was modified to not allow processes to use a pid of -1 unless they'd specifically set a process flag that they intend to kill all processes first, to help with programs that didn't check for errors when finding the process id for the singular process they wanted to kill.