I'm like 90% sure that's on Spotify. Apple opened up APIs to do this and Spotify refused to do it because... reasons.
The reason is that Apple wants a cut of the Spotify subscription money when users sign up for Spotify on an iOS or iPadOS device. IMHO, it's a bad look for Apple. They should either get out of the music business or stop taking a cut of the money from apps that compete with Apple services.
Do you have a source for that? As far as I'm aware, that has nothing to do with it -- e.g. Spotify is allowed on iOS as it is.
I haven’t looked into it in a while. I guess it changed between 2019 and 2021.