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.
Likely because one of the backend systems that implements this functionality depends on some deprecated service. And nobody cares enough to port the POP checker to use something new.
As a user of the feature, this is supremely annoying. They didn't even send me a warning message it will be discontinued.