That has never been policy at WDW.
My DD can't stand unless she is held up and has been riding all sorts of attractions since she was tall enough.
Someone who can't walk will need to wait to be evacuated, but they won't prevent anyone from riding just because they can't walk.
DD does not ride the Mountains or Rock 'n Roller Coaster, but has been on Tower of Terror, Splash Mountain, Test Track, Kali River Rapids, Star Tours, Mission Space, Soarin' (ones with height requirements I can think of off hand).
There are 2 at WDW that say "must be ambulatory' - Peter Pan and Peoplemover.
If someone is physically capable of standing to be measured, CMs will ask that that they stand. If they can't, the CMs will take the parents' word for the height as long as the child doesn't look too short.