They're an even bigger thing in emails. Usually, companies would include a pixel from an external resource and then figure out if you've read the email or not by looking at was that image loaded or not.
IIRC, GitHub does that. If you read the email of some notification, it won't show you that notification in your notifications. The solution is to block external image loading in your email client (I know that Thunderbird has that, and I know that Zoho's email client on Android has that).
Yep, tracking pixels are among the oldest forms of internet surveillance, predating the far more aggressive and intrusive JavaScript companies use today. They're one of the reasons why most mail clients don't load images by default. They only give information on page loads, not obsessive behavior tracking, but they're harder to block.
They're why I block doubleclick.net, google-analytics.com, etc. in my hosts file rather than just blocking their JavaScript.
Gmail does something to divert it - it downloads all the resources once and CDNs them for you, so the email author can't see who and where opened the email.
IIRC, GitHub does that. If you read the email of some notification, it won't show you that notification in your notifications. The solution is to block external image loading in your email client (I know that Thunderbird has that, and I know that Zoho's email client on Android has that).