Plastic backed adult diapers are still made.
You can buy plastic backed adult diapers online such as through Abuniverse and even other medical supply stores online search for plastic backed adult diapers or poly backed adult diapers.
Attends makes a plastic backed adult diapers and even Cardinal adult diapers are plastic backed.
https://www.carewell.com/product/simplicity-adult-diapers-with-tabs-moderate/
Northshore mega max adult diapers are also a plastic backed adult diaper.