The Google Developer Consoles (even the old version) no longer lists the Mail API in the list of available APIs.
I'm trying to develop a web application which acts on behalf of users to manage their Gmail, like Mailbox. This requires using the Gmail IMAP API with XOAUTH2 authentication with an OAuth 2.0 access token. The documentation instructs provisioning OAuth 2.0 token with a https://mail.google.com/ scope. Provisioning credentials for a web application then requesting a token with this scope yields a 403 response with an accessNotConfigured error. There are options in the developer console for configuring other APIs, but there is no option to configure access for the Mail API.
Attempting to provision the mail scope in the Google OAuth Playground using their client credentials works correctly and the resulting access token can be used to access Gmail IMAP with XOAUTH2.
Have Google disabled OAuth 2.0 authentication for Mail using new credentials, or is it just hidden? I can't find a way to contact them and ask for Mail API access.
Just create any client id it doesn't matter what API you select. When you make the authentication request send the scope of https://mail.google.com/. That will get you the access token you need to login to the imap server.