> My desktop is pure v6 and GitHub works fine, which I think disproves the "never" part.
how?
how?
$ dig A +short github.com
140.82.112.4
$ dig AAAA +short github.com
64:ff9b::8c52:7004
$ dig AAAA +short github.com @2a01:4f8:c2c:123f::1
2a01:4f8:c2c:123f:64:5:8c52:7004
2a00:1098:2c::5:8c52:7004
2a00:1098:2b::1:8c52:7004
Note the embedded 140.82.112.4 in the last four bytes of the v6 addresses, which you can write in v4 format if you want: $ getent ahosts 2a00:1098:2b::1:140.82.112.4
2a00:1098:2b::1:8c52:7004 STREAM 2a00:1098:2b::1:140.82.112.4