1996 said:
When did the movie soundtrack song become irrelevant? The last one I remember was Batman Forever with Kiss from a Rose and Hold Me, Thrill Me... I had so many soundtrack cassettes as a kid
They are still relevant but instead of being a bunch of soundtracks making medium impacts its a smaller number of soundtracks making a huge impact. There's still a place for soundtracks but in the Spotify age artists aren't limited to releasing new singles on some sort of album so there's probably less of a desire to lend out their songs to help out a movie if they can just as easy release it themselves.
A Star is Born, Spider-verse, Greatest Showman, Hamilton, Black Panther, Trolls, Pitch Perfect, 50 Shades Movies, have all done really well recently.