If there were any retention contracts that came with GitHub acquisition, that probably didn't apply to him.
The initial conversation was about how Nat was likely leaving because his contract ran out. The counterpoint was that he was from the Microsoft side and therefore he didn't have a retention contract. I brought up Xamarin because he was likely under a retention contract from that acquisition.
That being said, these contracts probably had little to no effect on his decision though, as I'm sure he would have made more money than he could spend in a lifetime regardless of whether he had stayed or not.
Microsoft bought Xamarin in February 2016. I'm sure five-year retention contracts are possible, but that seems extraordinarily long; I've rarely seen longer than two years.
I do not see any indication about that being bound to a contract. They are also promoting the chief product officer which indicates that he did so far a good job. Which means, we can expect that they continue like they have done in the last year.
So exactly what GitHub tried to do before the acquisition?