Not totally sure, but it's definitely not as active as the first three I mentioned. The last commit is from over a year ago [0]. Although the SML/NJ website shows releases presumably from 2020.
http://smlnj-gforge.cs.uchicago.edu/scm/viewvc.php/?root=sml...
A bit unfortunate as recent releases have fixed one of the main for smlnj/reasons to use other compilers in that it now supports x86-64.