Github hasn't had much time yet to respond to the various recent gripes about unfixed stuff. You should first give them six months to get their act together. You're making a rod for your own back if you go to all the trouble of investigating moving your project now.
Why not just demand an update from Github about which of your issues they are fixing? If you get no response then start investigating options.
"Respond" in what sense? If GitHub has any plans to address the issues outlined in the "Dear GitHub" letter, there's nothing stopping them from writing their own letter saying "We hear you, we're working on addressing the following concerns, and it's going to take approximately this long before you see results". Unless I've missed it, there's been no response at all from GitHub; if they care, why don't they say so?
Github has been aware of the gripes for a long time. The "Dear GitHub" letter came after many months of trying to get GitHub to respond through other channels.
The "6 months" timer doesn't reset every time you ask.
If GitHub wanted to do something, it shouldn't have required the "Dear GitHub" letter, and once it got to that they should have responded promptly. That they haven't ever responded official (to my knowledge) is a sign that they have no interest in dealing with the issues.
Why not just demand an update from Github about which of your issues they are fixing? If you get no response then start investigating options.