At that point, if such content is already posted there and available for download, it doesn't matter if it is only allowed to be downloaded via clearnet or VPNs as well. Blocking VPNs doesn't make any difference here.

My understanding is that it's for tracking/reporting purposes, and to mitigate future offenses by banning those IPs. You can report an IP to an ISP for CSAM violations, but it's not as useful when the user's on a VPN.

I've seen a debate regarding lemmynsfw with some people asking to turn off caching/proxying for images. I don't know what's their current status on this, but on my instance even thumbnails were not visible for catbox images. I'm not sure if it's disabled or it's the instance server itself having trouble accessing catbox.

Yeah, I've also noticed that Catbox links don't seem to generate previews on Mbin, as well, so I suspect that may be a Catbox block of some sort. That's interesting... I wonder if that causes a Lemmy instance to attempt a live preview instead of giving you a cached one. If so, that seems like something that probably shouldn't be in place, IMO.