The worst part is that there is nothing you can do if the person you are texting has enabled iCloud backups. There is also no notification when you start the conversation that it is not e2e protected anymore.
Ideally it would be nice if you could opt yourself out of having any conversations backed up, but I'm sure to Apple the privacy benefits doesn't outweigh the amount of customer support hours that would be wasted explaining to people why some of their conversations aren't transferring to their new iPhone.