How much github is secure?

First of all, your code is almost always backed up, because everyone in the project have their own local copy of it. So, if Github suddenly stops working, you can just share your existing code through another repository (like Gitlab) without lossing your commit history.

About the project metadata (issues, PR, comments, etc), according to Github Help [1], you can:

1. Use the Github API to retrieve metadata (it's pretty useful even if you don't want to make backups but just customize your IDE, see [2]).
2. Use a third-party tool (I suppose they use the Github API behind curtains), like BackHub [3]

[1] help.github.com/en/articles/backin...
[2] asylum.madhouse-project.org/blog/2...
[3] github.com/marketplace/backhub