Oh wow, I ended up here while digging into the related RSPEC documentation removal.
The endpoint I originally found broken is the separate sonarsource.github.io/rspec site, but I see from the official statement above that both the rules site and RSPEC repo were affected.
The scale of the broken references is pretty surprising. rules.sonarsource.com is now NXDOMAIN, while GitHub code search reports about 13,760 files referencing it in total. Only 20 of those are in the SonarSource org.
On the RSPEC side I found a similar situation: 923 occurrences across 626 committed files in SonarJS, plus about 420 files outside SonarJS referencing the public RSPEC site.
I posted the detailed RSPEC findings and one possible alternative here:
And the eslint-plugin-sonarjs-related breakage here: