The verbiage suggests that this will only apply to future uploads once iOS 15 is installed -- i.e., your 200+GB won't be re-scanned on your device, unless you download and re-upload your entire library.
Right - but I don't want to lose the access to that 200+ GB of backup (turning off iCloud photos) just so Apple won't use this system on my phone. That's what I did a poor job of saying the first time.