I don’t even understand what a capability based language is, but presumably closed source software wouldn’t apply here? Is it common for closed source software to give the source code to customers to build?

> Is it common for closed source software to give the source code to customers to build?

Yes, somewhat common. I work in embedded, and where a supplier gives us the ability to build their code it is a lot easier for us to deal with the next time we need a new build from them.