What happened to the "master password" feature to login, as was on the 1Password 7 app?

ChunkTuna "Master Password" and "account password" are synonym in the membership/cloud-stored model of 1Password 8. The term "master password" was renamed to "account password" with v8, and they are the same thing. If you used two different passwords with v7, it might be you just accessed some local vault with what you call master password and never actually unlocked access to the cloud-stored membership vaults. This could explain the issues with migrating you mentioned.

In v8, there are just cloud-stored vaults, no local vault any more. If your account password is too difficult to type, change it to something more easy. It's not productive to choose a too difficult password you're not able to type in just for the sake of security. That's actually lowering security (why: you will start avoid using the app)

Signing in on new devices still need the secret key, so a more simple account password doesn't lower account security. See https://support.1password.com/strong-account-password/ how to choose a good account password and notice the "your account password should be memorable" paragraph.

The expected workflow for daily usage is to use the biometric unlock for your devices instead of the account password. For Windows, it's Windows Hello and the Windows Hello PIN. Activate Windows Hello and TPM usage in 1Password and make sure you properly initialized your Windows Hello Pin within Windows. If activated and it's working properly and using the TPM, you have to enter you account password just every 2 weeks to make you not forget that password, otherwise you're able to unlock just with the pin. That pin is secure, because it's just stored locally, can just be used locally, and never shared across any network.