And how was it privacy compromising?

1. Anything could be added to the hashes with the user having no way to know what's being searched for beyond "trust us". This could be partially alleviated if, for example, the hash had to be signed by organizations in a combination of states that'd make it difficult to push through hashes for anything other actual CSAM (so not just Five Eyes)

2. Adversarial examples to intentionally set off the filter were demonstrated to be possible. Apple made it clear that there are types of content they'd be legally obligated to report once they became aware of, and it'd be well within a government agency's capabilities to honeypot, say initially, terrorist recruitment material

3. Coincidental false positives are also entirely possible (ImageNet had some naturally occuring clashes) and can result in their employees seeing your sensitive photographs

4. The user's device acting against the user cements other user-hostile and privacy-hostile behavior. "People could circumvent the CSAM scan" would be given as another reason against right to repair and ability to see/modify the software your own device is running

5. Tech companies erode privacy by flip-flopping between "sure we're giving ourselves abusable power, but we'll stand up to governments pressuring us to expand this" and then "well what were we supposed to do, leave the market?" when they inevitably concede