At least part of the reason that individuals traveled freely is that only rich individuals traveled.

Paupers from London didn't get on ships to the new world - not without selling themselves into indentured servitude.

When the economic barriers to migration fell to the point that subsistence farmers could travel between continents, we were always going to see either an equalization of living standards or the rise of political barriers.

Yes.

Even before the adoption of visas, the traveling experience for a first class passenger on a ship was very different from third class getting routed via Ellis Island in the US.

Not in the majority of Europe, especially places like England or Poland which had strict restrictions on what serfs could do for hundreds of years.

Part of the reason for Britains diverse regional dialects is due to this.

That was within the country though, right? Not between countries? And this article [1] says enforcement of serfdom was localized and costly and there was de facto serf mobility.

[1] https://broadstreet.blog/2021/05/07/serfdom-the-state-and-th...