NT could have used Texas Stadium to meet the stadium size requirement. Some of our home games were played there in the late 70's and early 80's before 20K seat Fouts was expanded, but the paperwork didn't get submitted in time (President Hurley), so we were forced to drop to 1-AA.
Here is the current requirements (an interesting read):
"The requirement began in 1978 when Division I split into what are now known as FBS (1-A) and FCS (1-AA). To keep together the schools that invest heavily in football, an FBS requirement was set with an average of 17,000 fans once every four years and a 30,000-seat stadium, among other requirements regarding scheduling, scholarships and other issues.
In 2002, the football oversight committee changed the requirements to what they are now: a 15,000-fan average, with no stadium size requirement."
https://theathletic.com/1070204/2019/07/11/college-football-ncaa-15000-attendance-rule/