Email isn't much worse at leaking metadata than most of the things people use for messaging and is better than many:

* https://articles.59.ca/doku.php?id=em:anonemail

Ultimately, for the strongest privacy protection you need to go to something offline, like email:

* https://articles.59.ca/doku.php?id=em:emailvsim

Obviously not everyone needs the highest level of protection, but the fact still needs to be acknowledged.