I was afraid this would result in having to rebuild the JS. Thank you for tackling this @ideag !

Happy to help!
I'm still not sure what to do with the dotorg url in the actions part. It's so generic it seems better to just remove it.
I also hate how I have to regex the error message in, but I see no way around that at the moment.

I've updated the regex to support translations as well. Decided to leave the actions link to dotorg as is for now. We can revisit if/when we have a different link to use there.

Great work discovering the JS approach!

I wonder if we could simplify this, by leveraging the rest_post_dispatch filter? That would allow us to combine all the logic in a single PHP filter.

Optionally, filtering the site_status_tests and removing the one for ['async']['dotorg_communication'] and replacing it with our own, but then we need to also create the rest lookup and such. It depends entirely on how much control we want over the entire lifecycle of the test.

I thought about that, but using a purpose-specific filter seemed cleaner to me. Moving this to rest_post_dispatch also does not solve the need to parse the API response with regex, so it doesn't seem to be an improvement in that respect.
Converting this to a non-async check is an option, but a more involved one. I assume dotorg did it this way with performance in mind and I agree with that judgement.

Filtering the site_status_test doesn't restrict you to only doing synchronous tests, it can be added as a new async test, it does provide the most control, and retains the performance intent behind the original implementation, but does mean we need to write the whole response our selves (but that may be preferable to just conditionally replacing the strings of a response?)