I’m not a historian but I would have thought that open borders were the norm not that long ago. I would guess that immigration restrictions wouldn’t have started to be commonplace until at least industrialization.

But regardless if that is right or not, siblings have pointed out that open borders are in fact common-place in many parts of the world today, e.g. EU and USA’s internal borders are open. And (as sibling has also pointed out) historic precedent or existing examples are not necessary to justify any belief. Abolishing slavery did not need a historic precedent, neither did gay rights, and theoretically neither does open borders.

Many of the immigration restrictions in america started around the time the first welfare programs were introduced. Before that, it was a bit of a free for all. I don't know if the same pattern holds anywhere else, but I'm suspicious that might not be a coincidence.

Isn't every human right without historical precedent at first, until it becomes established?

Doesn't that always happen step by step, with "early adopters" arguing on grounds of morality and other arguments, until eventually enough people start to believe something has a high moral value and should be raised to the level of a human right, enshrined in law?

Those who would argue for open borders as a human right are like "early adopters" of that idea.

There would be consequences for that. For example you would need a universal world wide social safety net.

We would all fit easily on Iceland, but most would die of dehydration or starve in a short amount of time.

I could be an early adopter of "cheese for everyone", but I would need to get a lot of cheese first to make that happen.

World wide safety net: Open borders isn't the same thing as everyone having the same safety net in whichever destination they travel to. Though related, they are distinct issues that can exist independently.

That said a world wide safety net would be a good thing to advocate for on moral grounds, no?

The EU has open borders for all EU citizens, and for the most part it's faring ok. There has not been a Europe-wide social safety net crisis as a result of this freedom, and crucially for this argument: there isn't an EU wide social safety net. It has freedom of movement without a universal safety net.

The USA's states are effectively countries. All USA citizens have open borders to travel between states in the USA. For the most part, it's faring ok and there has not been a USA-wide social safety net crisis caused by this freedom either.

Iceland and cheese for everyone: That sounds like what's called the slippery slope fallacy. https://en.wikipedia.org/wiki/Slippery_slope

But I believe most participating countries in the EU have pretty strong social safety nets. And the US' social safety net system is at the federal level. How would a world safety net be financed?

US states are not countries.

That was set with our Constitution, and thoroughly tested in the 1860s. The idea has never really been seriously tested again since.

It’s a quite different setup from the EU.