The main principle here is to reuse as less infra as possible: different dns provider, different hosting, different cert authority, etc. So, debug.github.com is probably worse, than github-debug.com (github.debug should be fine though, and even slightly better than github-debug.com)
You could use two independent dns providers for the same domain (i call it inverse split horizon). I am a proponent of dns service discovery (using NAPTR and SRV records) so that by going to e.g. www.github.com, the dns provides a list of servers, and the final server would be your debug site (kind of like how there are MX records with different priorities, except naptr/srv is service agnostic).
Hi, I assume you're the writer or at least work at Dropbox.
Just want to let you know (as I mentioned somewhere): this blog link doesn't load at all if I click from here (i.e. with the refer of news.ycombinator.com).
To isolate possible issues with gtld, dns + glue records, registrar, domain blocking (corporate firewalls, restricted/compromised user dns, malware) that affects .com (the github.debug that is, debug.github.com is not a good solution for the above reasons)