>> Lanterman-Prentis-Short Act of '67 capped the length of involuntary commitment half a decade before the SCOTUS decisions

Mostly agree with your argument and I'm a Reagan fanboy, but Reagan did put his signature on that law. It was a bi-partisan bill (Lanterman was a Republican, Petris and Short were Democrats), and it passed with veto-proof near unanimous majorities, but he put his name on it.