The API was an Echo Nest API. Spotify acquired Echo Nest and shut down all their APIs in 2016.
I'd love to see the app live on too! Spotify's API has similar functionality[1] to the old Echo Nest API, for now at least. (But I don't know if it returns all the same data.) Or, if you don't want to rely on Spotify, I bet Essentia[2][3] could do the job just as well. Essentia is the open-source brains behind AcousticBrainz[4]. So you could either use Essentia directly, or grab the data from the AcousticBrainz API.