Here’s the thing: With the introduction of recovery keys, we didn’t change anything on the masterkey file itself.
Each vault has its own 256 bit encryption as well as MAC masterkey used for encryption of file specific keys and file authentication, respectively.
Since nobody can remember 256 bit keys, you can choose a password. According to Cryptomator’s encryption scheme, the keys can then be derived from a user’s password using scrypt.
Important: The 256 bit keys never change for a vault. Even if you change the vault’s password, the new password still derives to the same keys. If we’d change the keys, we’d have to re-encrypt all files inside the vault.
A recovery key is just another way to derive the 256 bit keys. Okay, that’s not technically true, a recovery key is another representation of the 256 bit keys.
(If I’m not completely mistaken, the recovery key consists of two 256 bit keys, encryption + MAC, maybe we should document that somewhere as well, but that’s not the point.)
What does that mean? If you never show the recovery key, there is nothing to worry about. The 256 bit keys are still secret and can only be derived from the only password you’ve chosen. How should anyone decrypt your vault with a recovery key if it has never seen the light of day? Basically, you can’t opt-out of the 256 bit keys.