Tracking pixels.

I suppose you're aware but all the pixel does is tracking whether you open it or not since Google preloads and proxies it. It's never served from the original server.

I like opting in to read notifications. Using a tracking pixel for this is a privacy violation (albeit minor).

Yup, fair point. Still wanted to emphasize that the tracking pixel is way less intrusive than it used to be when you're using Gmail nowadays.

Correct, that level of tracking alone is enough to make me want images off by default.