This is an interesting one. Doing "whois github.com" the nameservers returned from whois.markmonitor.com were at one point:
Updated Date: 2020-06-22T12:02:10-0700
[...]
Name Server: ns-1707.awsdns-21.co.uk
Name Server: ns-1283.awsdns-32.org
Name Server: ns-520.awsdns-01.net
Name Server: dns3.p08.nsone.net
Name Server: ns-421.awsdns-52.com
Name Server: dns1.p08.nsone.net
Name Server: dns4.p08.nsone.net
Name Server: dns1.p08.nsone.net
Name Server: dns2.p08.nsone.net
DNSSEC: unsigned
URL of the ICANN WHOIS Data Problem Reporting System: http://wdprs.internic.net/
>>> Last update of WHOIS database: 2020-06-22T12:23:58-0700 <<<
Then as of a bit later:
Updated Date: 2020-06-22T12:26:23-0700
[...]
Name Server: ns-1283.awsdns-32.org
Name Server: ns-1707.awsdns-21.co.uk
Name Server: ns-520.awsdns-01.net
Name Server: ns-421.awsdns-52.com
DNSSEC: unsigned
URL of the ICANN WHOIS Data Problem Reporting System: http://wdprs.internic.net/
>>> Last update of WHOIS database: 2020-06-22T12:34:32-0700 <<<
The problem is:
$ dig @a.gtld-servers.net ns github.com
[snip]
;; AUTHORITY SECTION:
github.com. 172800 IN NS ns-520.awsdns-01.net.
Those records have a 48 hour TTL. So while this works around the problem with nsone, if they can't fix those servers it might be a while or need manually flushing caches before github.com fully comes back for everyone.
[edited to add updated date]
Now mentioned at https://www.githubstatus.com/incidents/shdhxntw519h