I want to create a website that allows users to generate an ERC-4337 compatible Ethereum wallet using just an email address. 

Is there a guide for this? Any good examples in the wild?

Elevating onchain creators @brightmoments @ampsfun @nodefoundation

All current methods that do this use an app-specific OAuth key to create the signer- meaning users will need to use the same app as the one they used to create the wallet to access their wallet in the future

The wallet is not universally accessible through any app that offers email login, it’s a centralized solution

That's ok. You can still transfer assets out of it.

I think it should be fine as long as we all use 4337 account contracts that allow adding other owners to the contract, so other apps, devices, logins, etc can be given access without needing to create an entirely new account contract.