I have long since switched to only doing local encrypted backups, but for some reason it never clicked that of course all of my messages are included in other people's backups. Frustrating that its E2E with a bunch of caveats.
I'm not sure how E2EE came to be interpreted as to mean "totally secure against everything".
Your messages, phone book, pictures you share with others etc. are still 'readable' on the remote end and thus still get collected. And if you connect the dots when you have a large collections your personal data can be reconstructed from that.
If you have persons A, B, C and D in your phone book, but your phone book is 'secret', it doesn't prevent someone from knowing that you know A to D if those still have you listed.