You don't need to break TLS to do IP/domain blocking, but you can't redirect an https page unless you have an acceptable certificate.

> but they're only broken out for big investigations or secret dragnet stuff we'll find out about in five decades, if ever.

Certificate Transparency, where required, makes certificates unusable if they're not published... But that might not be enough information.

You certainly can, but you should get a big screaming "this site's certificate is not valid for dodgy-casino.games" warning.

If not, then maybe your browser vendor has been pressured to add some root certificate controlled by the Serbian police, which it approves to issue certificates to impersonate dodgy-casino.games.

This is a DNS hijack, not an HTTPS hijack. The ISP's resolver sees "casino.org" in the A/AAAA query, finds it in a blocklist, and responds with an IP address to a web server that serves a block page (or a CNAME thereto).