This means that whenever a user visits a website on Cloudflare that has ECH enabled, intermediaries (such as ISPs, governments etc.) will be able to see that you are visiting a website on Cloudflare, but they will not be able to determine which one.

Which also means people who lives under censorship can circumvent the censorship.

Since e621 using CloudFlare supporting ECH should be pretty easy by enabling it. -> https://developers.cloudflare.com/ssl/edge-certificates/ech/#enable-ech

When its enabled https://e621.net/cdn-cgi/trace should show sni=encrypted instead sni=plaintext

You'll also need an encrypted DNS for this to be useful, otherwise you're still sending plaintext DNS requests (which contain the domain you're connecting to), which are pretty trivial to censor (and spy on, of course).