What is the crypto keychain, and why do I suddenly have an app that wants access to the password and won't take deny as an answer

Apple has nothing called Crypto Keychain. Whatever that is, is not form Apple. Apple has a Keychain, an app where it stores all its passwords, which is encrypted. But has nothing to do with Crypto, which is digital currency, and would be normally stored in a digital wallet app if you use such things. Apple does not store digital currency in its own Wallet app.

Why is a streaming app requiring access to it? Only the app knows. Perhaps it needs a password for something.

If it's not actually requiring access to a Crypto Keychain, but more likely requiring access to your Apple Keychain, then it likely needs a password from there. However, if you don't trust the very app you use to stream things, then perhaps it's best you uninstall it.