That's true, though I avoid this problem by only opening HTML email from known, trusted senders where I consider it acceptable to signal that I opened the email.
Unfortunately even "trusted" senders I know often like to use third-parties to handle tracking and analytics and from there you don't really know where it's going to end up.
Of course you might argue that I shouldn't trust people using these services but then I might as well stop using the internet altogether.
For instance if I open some marketing email I got in the ProtonMail web client I get a warning that "This message contains remote content" with the option to allow it. Before that my browser makes no query to any third-party website so nobody can track it.
If I open the same email in firefox from mutt, firefox immediately fetches the remote content normally and unconditionally.
You could probably script something to start a browser in offline mode but I haven't seen most people do it (see the replies in this thread that just tell people to "open HTML with Firefox"). So paradoxically mutt users might be easier to track than ProtonMail, GMail and other HTML-aware webmail users.