I would just like to mention, that in my opinion "decentralized GitHub" is an oxymoron, Github's innate feature is being centralized, to centralize developers into one place, being a kind of social network.
Do ad hock networks lead to fetlock-in?
A hock is a joint in the lower end of a quadruped's hind leg, between the knee and fetlock. It's in analogous position to the ankle in humans.
Of course 'hock' can also mean 'to sell, especially to a pawn broker'.
The word you want is 'hoc' which is Latin for 'this'. You may be familiar with the 'post hoc' fallacy: which is that if event A is followed by event B, event B must therefore be caused by event A. The full name of the fallacy is 'post hoc, ergo propter hoc', which means 'after this, therefore because this'.
The latin word 'ad' simply mean 'to' or 'for'. So 'ad astra' means 'to the stars', an 'ad hominem' argument is to literally direct your argument 'to the person' and their faults.
So finally we arrive at the combination 'ad hoc', which means 'to the purpose'. Ad hoc things are done in purely pragmatic manner, for a particular purpose. They are often short lived arrangements which end once their purpose has been meet or is made moot.
Oh, and you didn't make this mistake, but I'm going to blather about it anyway. 'etc.' is short for 'et cetera', it is not spelled 'ect.', nor 'excetera', nor is the usage 'and etc.' valid. 'et cetera' means literally 'and the rest'--meaning anything, not necessarily just the Professor and Maryanne.
The ad-hoc I was reffering was : https://en.wikipedia.org/wiki/Wireless_ad_hoc_network
And I am aware of "etc." ;)
GitHub is a center for activities: PRs, issues, notifications. It's only a matter of convenience. To be successful you have to create an equally (or more) convenient decentralised alternative.
If you have a tight knit group of friends getting everyone to swap over to Mastodon (and slowly bring their friends) isn't that difficult. That's how the social effect works whenever there's a max exodus of users from Platform A --> Platform B (like what happened with Myspace-->Facebook, Digg-->Reddit, etc.). It isn't impossible - just unlikely.
You can see some statistics here: https://mnm.social/
Do you believe internally Github is a single big server? And that big server only have a single HDD, a single chip of ram and a single core? Even then I'm pretty sure we could argue there's some separation there. It doesn't means the interface isn't centralized.
Same goes for Bitcoin. We can all access the same data, yet it's decentralized.