If large parts of the planet lose their digital infrastructure for months, I really don’t think that “finding a good platform to host my code” is going to be one of my biggest problems.

Even if that's a concern in a doomsday scenario, self-hosting gitlab is super easy and a good (some would argue better) alternative.

At that point the enormously powerful central players like big tech and militaries and tax collectors will be more than incentivezed to use every remaining resource they have to bring everything back online and re-centralize power. And if they can't - society and power wasn't exactly more distributed in the distant past lol. Your local warlord/military dictator/whatever will probably not be supportive of nerds acting independently.

You've described a scenario disastrous enough that my primary concern would be drinkable water.

ok fine, ill thank the nerds in that case.