Google nowadays just seems afraid to store personal data. Google have moved your maps history/timeline to be on-device only, seemingly in response to geofence warrants. They're also deprecating health (Google Fit) APIs, and moving that data to be on-device.
So, removing POP (where they need to download emails to their servers), and only supporting IMAP (where emails stay on the third-party server) via their GMail app, that would be consistent with a policy to store as little personal data as possible. (it could also be completely unrelated :)
IMAP was never supported and is still not supported for external account un gmail web. And they don´t remove pop. You can use pop to access your gmail in any email client or in the app if you wish.
So, removing POP (where they need to download emails to their servers), and only supporting IMAP (where emails stay on the third-party server) via their GMail app, that would be consistent with a policy to store as little personal data as possible. (it could also be completely unrelated :)