Some users have embedded images disabled in email client by default.
some users have javascript disabled, some have email clients disabled. it's not foolproof but works for the majority of users, i.e. >99%
Any any well designed website will work with javascript disabled unless it is absolutely needed for the task (i.e. we are talking about an actual app).
There is also no guarantee that the image being loaded means that the user opened the mail and it wasn't just preemptively cached by the server/client. There is also no guarantee that the user opening the mail means they read it.
have you seen the web lately? javascript is increasingly being used on backend. not just for apps, for static one page sites too. those who disable javascript do not have a pleasant browsing experience and are forced to enable it often
I'm not here to guarantee anything, just mentioning the technology exists and is still widely employed. there are no guarantees with messaging app ticks either
Are you sure those are GDPR-compliant? Just because you can do something doesn't mean that you should.
that's what those embedded 1x1px images are for