Sources for archive.is blocking Cloudflare's datacenters:

The exact same command fails when sent from Cloudflare's datacenters, but succeeds when sent from DigitalOcean:

https://community.cloudflare.com/t/archive-is-error-1001/182...

Two more sources:

https://news.ycombinator.com/item?id=19830258

https://news.ycombinator.com/item?id=19829036

I've seen that, it doesn't really clarify whether the block singles out cloudflare in particular, or whether cloudflare is the only (significant) DNS resolver that the block happens to affect.