“backdoor” is hyperbole. I think EFF is a great organization, but that doesn’t stop them from overstating things from time to time.
A backdoor gives unfettered access to much or all of an operating system to allow things such as keyloggers, stealth access to the cameras and microphones, detailed location data regardless of your settings, and collection of data that even you don’t have access to. Nation states and a few others can already do this by taking advantage of bugs, e.g. Pegasus.
Apple’s CSAM has a mechanism more akin to downloading google’s safe browsing database so it can block you from inadvertently going to a site that might be hosting malware. I’m a little surprised that so many people on the net who claim to be security aware would be using iCloud photos or iCloud backup anyway, because they’ve never been end to end encrypted. If the feds want to see your photos in iCloud Photos, all they need is a subpoena. [Photosync is great way to backup your photos to your desktop or to many other destinations. Don’t send them to Dropbox though–they scan with CSAM…]
There is no slippery slope here. Apple has always had the ability to use a software update to change how it accesses your data because they wrote the whole system. They make changes we do and don’t like all the time. At least for CSAM you can opt out of having your photos checked on device at all by opting out of iCloud Photos. Many other services don’t let you opt out of it at all.
Whether it’s a good/bad/indifferent idea for Apple to be doing this, or whether CSAM is effective enough to be worth the trouble, are different issues and I don’t know enough to have an opinion. It’s certainly brought them bad press. Of course the bad press that they really need to have aimed at them is that they reneged on their promise for end to end encryption for most iCloud services, but for some reason the press just shrugged ‘who cares’ at the far bigger sin.
Examining photos via messages with ML is a whole 'nuther thing unrelated to CSAM except by proximity of announcement. Since it’s completely opt-in by the parents, I fail to see why that would be alarming to most people. But I’m no longer a kid, and I’ve never been parental.