Unfortunately, standardization != widespread use. Tracking pixels are still the most reliable way to know whether a recipient (or many) opened your message.
I can think of one email client with ~15 million users that does that.
https://support.google.com/mail/answer/145919?co=GENIE.Platf...
K9, gui Android (think it blocks by default, or I turned off auto load when I installed ages ago).