MX Player offers a vast library of movies and TV shows completely free of charge (ad-supported). It hosts a significant number of older titles from the 80s and 90s.
Shemaroo is a legacy distributor of Hindi films. Their app and website have a specific "Bollywood Classics" section. While some content is premium, they often have free sections and their YouTube channel is very active. Old Hindi Movies List 1980 To 2000 Free Download
Several apps offer legally free streaming of old Bollywood movies with advertisements. This is the safest way to watch without piracy risks. MX Player offers a vast library of movies