https://soundcloud.com/you/apps/new
They don't seem to be accepting new applications. I couldn't find much information other than this tweet.
https://twitter.com/SCsupport/status/1053216471818665984
Just guessing here but I think it’s due to the financial troubles SoundCloud had in summer 2017. They had to lay off half of their staff and closed some offices in SF & London. Wouldn’t be too far fetched to assume they scrapped their API to save some money.