This is a much more reasonable solution thatthan simply deleting them, true. However, there are still quite a few problems with it:
You are still moving this content off-site which is part of why we don't want to delete them in the first place.
Many users will be scared off by github. This is AU not SO. This means that your stated goal of allowing edits to continue will likely fail.
The answers will no longer be indexed by google so they will stop bringing in views which is one of the main reasons we want those crappy questions to remain.
Why in the world would anyone want to go to such a huge effort as setting up a whole github repository when the questions and their answers are already here on the site? Again, we're not SO, we're orders of magnitude smaller.
I am still not convinced that there is a real problem here. The only argument against keeping these questions I can think of is that they might (I said might) give new users the impression that such questions are on topic. I don't see evidence of this. While we are indeed flooded by a metric crapton of bad questions, I don't recall seeing any that asked for a list in the past few months. I really doubt that these questions are causing us any problems. They're closed for Pete's sake!
Which brings me to my preferred "solution" for this "problem". Simply close them and add a message to each of these questions under discussion that reads:
This question is present as a matter of historical interest. While you are encouraged to help maintain its answers, please understand that "big list" questions are not generally allowed on Ask Ubuntu and will be closed per the FAQ.
This has already been done for the IDE and media player questions. Let's do it for the rest as well. This approach has the following benefits:
It lets us keep the questions, and the traffic they bring, while making it very clear that such questions are not wanted.
The existing answers can still be improved but no new answers can be posted.
It does not require the (extensive) extra work of setting up an external repository for them.
We don't send users off to a different website to find the information they are looking for.
In combination, the above seem to address all of the issues you have with this type of question, we get to keep them and new users won't use them as an excuse to post similar ones.