Ok, I received a follow-up from support and it's working again.

This doesn't appear to be working. My team is reporting lots of problems with certificate errors with our hosted instance. It was working for me in Chrome as long as I was signed in, but opening a private tab I was redirected to admin.phacility.com and received an error there that the cert had expired.

This sounds great, thanks. We only plan to use it for testing Diffs at this time, so the other branch limitations aren't a concern right now.

Thanks for the quick work on this!

Thanks, there's an example here of the API:
https://buildkite.com/docs/rest-api/builds#create-a-build