@amckendrick I don’t have the exact details but if I had to guess I’d say custom extensions were sunset because of lack of usage (I don’t remember ever seeing a custom extension developed outside of Auth0).
In any case, the product is driven by customer needs so any feedback on the matter is welcomed.
Do you have specific ideas in mind? Is this something that you think would be beneficial to other users? If you want to leave feedback with a description of your use case at Auth0: Secure access for everyone. But not just anyone., this reaches the Product team directly and they will love to hear about it.