I live in Spain and can still enjoy access to this site. This is enforced by major ISPs (Vodafone among others), so anyone sane just opts for a local ISP (if they have a choice)

How is it enforced? IP or DNS?

Differs by the ISP you have, I've seen DNS manipulation, IP filtering, hostname filtering, and HTTPS interception so far. Obviously most websites use TLS today so it mostly "fails" so people see certificate errors rather than their scary "You're contributing to breaking the law blah blah blah". Sometimes just straight up timeouts, resets, or generic connection failures.

According to this site https://bandaancha.eu/articulos/gobierno-tiene-lista-web-ofi..., they use SNI (https://en.wikipedia.org/wiki/Server_Name_Indication)

Enabling ESNI in Firefox should be a countermeasure (encrypted SNI extension of the TLS so that hostname is no longer sent in plain text)

https://superuser.com/questions/1346634/modern-browser-with-...

It's not magic. archive.is has no HTTPS record, so ESNI cannot be used.

[deleted]

It also doesn't seem to be enforced on my Movistar connection, which means the single biggest ISP is opting out?

Nor on Vodafone for me, 3rd biggest ISP in the country. Guess that leaves us with Orange or possibly Digi as the second one that maybe actually has implemented the block?