* github profiles are rarely deleted and content put up once will stay there for many years

Seriously, dead links to github.{com, io} are super-rare IME.

Aren't they deleted if they're inactive for N years?

I don't believe this is the case. They may become eligible for reclamation if someone else wants the name, per the Name Squatting Policy [0].

[0] https://help.github.com/articles/name-squatting-policy/

Yeah, exactly this. If your article/gist/etc gains some traction it's an easy way to get people to look at your other projects. Github's success is largely dependent on the social aspect.