pert:
Of course we should always have our own backups but it's doesn't really seem worthwhile to maintain backup infrastructure to retain the ability to easily collaborate considering the decent uptime of these services.
If one can be confident that the website owner will fix the problem one can live with occasional interruptions in a non-essential service like this Forum or Github.
But the risk with having all one's eggs in the same basket (i.e. Arduino OpenSource code in an Arduino controlled website) is that the website owner may decide to close the website - for financial reasons or otherwise.
...R