Short video:

Question: Can I unsubscribe from the App Builder? If yes, what happens to the custom flows I have created?

You can unsubscribe from the App Builder at any time you wish to. However, when you unsubscribe, none of the custom flows will be useable. 

Click on Installed Apps

You cannot unsubscribe if you have any ExoPhones associated with the custom flows you have created. So, you either delete those ExoPhones or assign them to different apps.

You can re-subscribe to the App Builder any time you wish again.