You mentioned only unencrypted OS backups (which were a major issue, but also industry standard, affecting everything but Signal which takes a severe usability hit over it, and apparently fixed https://faq.whatsapp.com/general/chats/about-end-to-end-encr...). "Filtering content on the edges" is a whole debate but not something that ever materialized.

It sounds there's a list, what are the others?