I had a demo for some high-school students for an ethics and tech class that successfully demonstrated these with a GMail account, so when this started happening I got very upset lol.

So does Apple's Mail Client. So do most webmail providers. Open signals are generally worthless.

I think gmail adds some heuristics on top of it, like if the same image was included in emails to multiple people.

At least that's what I remember from them announcing the feature. No idea about other providers, and I haven't tested the feature myself.