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Last tested: Jan 21, 2019
If you disable SAML authentication, then all users who rely on SAML authentication will be locked out of the application. They won't be kicked out until their session expires, though.
This is based on SAML Docs:
Don't disable SAML authentication while you are logged into Looker via SAML unless you have an alternate account login set up. You could lock yourself out of the app.
However, any user with other login credentials (email/LDAP/Google) will still be able to log in using those authentication methods.