Been using Spotify for a number of years now, but it seems like a feature I used to really like has disappeared? You used to be able to pick one song and then tell it to build you a playlist based off of that song and it would pick some really interesting deep tracks and things you’d never think would work, but it would work

I don’t seem to be able to find that feature anymore, did they take this away? And if so, why?

On the song you want to do that, hit the 3 dots on the upper right and scroll down to "Song Radio"

There is also 'Smart Shuffle'
I use this on the 'Liked Songs' playlist, find some new stuff that way.