The reason is explained by Cloudflare CEOin the top voted comment at https://news.ycombinator.com/item?id=19828317

Archive.is is inaccessible via Cloudlfare DNS, see https://news.ycombinator.com/item?id=19828317

I was about to say "I don't use Cloudflare DNS" but eventually I remembered that Firefox's DoH defaults to that. Thanks.

Options -> Network Settings -> DoH provider

That’s because the owner of archive.is blocks DNS resolutions from Cloudflare DNS. They don’t actually use Cloudflare; rather, they have a grudge against Cloudflare, so they return an IP address belonging to Cloudflare when you attempt to resolve through 1.1.1.1. That makes it look as though it’s Cloudflare’s fault when it isn’t.

tl;dr archive.is wants Cloudflare to implement ECS, Cloudflare refused due to privacy concerns, archive.is doesn’t allow queries from 1.1.1.1 to resolve correctly.

I get a cloudflare http 403.

Seems to be working for me.

Works on my machine