Comment by JBerlinsky

Yes, developers can request permission to access your inbox "offline" (i.e. at any time, with a long-term access token). The scope is clearly identified in the OAuth authentication process, and the consent screen clearly asks for permission to (IIRC) "read, write, and delete emails." There are a few well-known applications (Earny and Unroll.me come to mind, off the top of my head) that are known to work with consumer research operations with the resulting data.