However, searching the ConTeXt source I find it used in enco-ini.mkiv, defined in syst-ini.mkiv, but commented out in syst-ini.mkxl.

Why is that? It seems a harmless definition, and its body is even used twice in enco-ini.mkxl.

I dare say, with all due respect, it is not pleasant (and a waste of time) to find working code suddenly broken because of leftout macro's. And I feel lucky to not having used \ialign, because something similar happened to it.