I do most of my non-work development on a non internet connected computer purely because I do it on a train. It isn't strictly airgapped, but I did develop an iOS game nearly entirely this way (with the very final steps needing to be done online, and also on an apple machine)
Things that I find:
- I develop primarily in C, so I have man pages for the stdlib accessible
- I try to rely on header files for documentation of other things, and find them to be far better than things like auto generated documentation or even manually written guides
- For algorithmic style things, I don't like to implement things without understanding, and to me that means being able to have them in my head to a good enough standard that they can be implemented without needing to be online. e.g. one of the sounds in my game is made by a physical model of a string, and while it is not the best synth that has ever been made (in fact, it is pretty bad), I definitely understood it enough to get something plausible without needing references
- There are things that I suspect would be quite difficult to do offline. Maybe they are just the things I am unfamiliar with, but I suspect that some project types are just harder to do offline
- You tend to accumulate useful documents on your Dev machine. Keep these organised as it turns out to be very nice to have on other machines. Language specs, file format specs, protocol specs etc