In the mid to late 70s, the concept of fitted diapers would be introduced, giving diapers their now iconic hourglass shape, with leg cuffs along the center edges to reduce leaking out the sides, and once the 80s came around, these diapers would also replace their absorbent sheets with a central core filled with super absorbent polymer (SAP), which absorbed liquids much better than any past diaper could! As the basis of all modern diapers were formed!

These diapers are iconic, as they were the ones that crinkled while looking the part!

And throughout the 80s more and more elements were added to aid the diaper in doing its job better, such as a barrier along the inside of the waistband to stop liquids or other 'messes' from spilling out the top, tape panels with cartoons on them which not only looked cute, but also prevented rip damage to the outer cover, and would also serve as a guide. And for certain diapers, additional absorbency in certain areas depending on the child, with more protection near the front for boys, and more protection near the middle for girls, sometimes color coded Blue and Pink respectively! (Although later diapers would become more unisex)

Come the 90s, leak lock barriers would be introduced on the insides of the diaper, further helping in preventing leaks out the side. Along with more diapers being made for different ages in consideration, such as for newborns, a special cut along the front allowed their bellybutton to heal without needing the top to be folded down, or additional protection along the waistband for infants to prevent their watery stools from blowing out the top of diaper while lying down.

The late 90s was also when the first size 6 diapers were introduced, for those kids not so ready to start potty training yet!
Rather controversial for the time, as Pampers actually ran an ad where a pediatrician advocated for how some kids may need more time before being potty trained. Due to how potty training should not be made into a chore for the child, he presented the 'Size 6' Pampers to help parents in not bullying their children into being potty trained.
-Quote from  Yosh-E-O https://www.furaffinity.net/view/45.....#cid:162294606

The ad in question:
https://www.youtube.com/watch?v=X-gJU33_QB0

Amazing as all these new standardized additions were, there was still the issue that these plastic backed diapers can be rather stuffy, and that's not a good thing for the more sensitive skin of babies, so come the turn of the new millenia, a new material would come to replace plastic as the main backing for diapers, almost replacing it in the modern day entirely.