This state's classification carries no date range for the species.
TPWD explicitly lists Mearn's (Montezuma) quail among the upland game birds with no open season, alongside Attwater's and lesser prairie-chicken.
Confirmed absence: Montezuma (Mearn's) quail has no open season anywhere in Texas (TPWD explicitly lists it among species with no open season), and no TPWD survey of any kind was found for the species.