The bigger programs (Bama, Oklahoma, USC, tOSU) used to hold massive practices with anybody and everybody. Bama, in particular, had country boys stashed at Shelton State who practiced with the team for two years before arriving on campus. Oklahoma used a similar ploy with one of the community colleges in Ok City. One of my father's friends played for tOSU in the early 1950's and explained that at that time, many of the football powers interpreted the practice regulations as meaning as long as players didn't practice with the "1st squad" and didn't suit up for games they could practice as much as they wanted, no matter where they went to school. He said that in 1952, the Buckeyes had 105 players on the roster, but practices sometimes involved 20 more players who were not enrolled as students. When the NCAA cracked down, they stopped calling them practices and started calling them "clinics." The NCAA, being what it is, began the process of granularly defining practice, eligibility for practice, practice limitations for coaches, etc.
The biggest change probably came with the Prop 48 stuff in 1983. My understanding from the compliance staff at SC is that waivers are not often sought they take up far too much time that could be spent on other compliance issues. For instance, if you are USC, why waste time on a waiver for a kid who probably will not qualify, when you have to run grade scams and PR campaigns to get Joe McKnight qualified again... not to mention the additional staff you have had to hire to hypnotize the entire @+**%#* country into forgetting that Reggie Bush ever played at USC.