Either someone forgot to renew a deal, someone messed up, or G4 just isn't a priority anymore, but it's looking like Friendship is Magic music is no longer available over on Youtube Music. Since a lot of people use this streaming service for poni, it's pretty unfortunate. If you visit the page for one of the songs, you will find the video unavailable. Over on Apple Music, the main page for MLP no longer lists the albums, though you can still listen to them if you have them.
This comes after a company called Magic Star took over for the distribution of MLP music across the various platforms available. Apparently they've had some issues in the past with typos, incorrect information, and general lazyness with copy pasted things being incorrect as they reupload albums.
Hopefully things are fixed up and we don't see another purging like Friendship is Magic saw over on iTunes! As with all of these streaming situations, be sure to have local backups of the files of stuff you really like, as you never know when they will delete them.