- Existing apps that are still in development mode without a pending extension request
- New apps that are registered on or after today's date
Most people writing small scripts were probably using an API key with development mode.
Yep, me included. Since my apps were never meant as anything more than utilities for myself, I never applied for extended access. Nevertheless, I used these tools multiple times a day, especially for sorting and filtering playlists by audio features such as energy and valence. Now, apparently, they will never work again. I'm sure there are plenty of other hobbyists in the same boat.