Ok, right, but some countries do ask that
Which ones? I've lived in multiple EU countries and none asked for a proof. There are even some countries that don't ask these questions at all. I could put in those forms whatever I wanted. And I believe if you moved on the 1st of January, there's no need for them to even investigate that since there's no risk of any
tax evasion or any other coordination with other countries since it's a dead simple situation. The year before you didn't live there, the current year you'll spend it all there. Officially.
And so you'll end up with 2 tax IDs, and they won't know about each other. But once it'll become known by some chance you'll have to sort out the mess with both the tax authorities. And so simply preemptively you'll start building a case for being a resident of the low tax country. Just in case it will need to be determined at some point in the future.
People own straight up
offshore companies, clearly committing tax evasion, whether they know it or not, for years, without repercussions. Many in the UK that has a public register of UBOs. So I'd say this information sharing is not at all impenetrable, it's more like a mine field, you just need to know where not to step.
Once I declared an employment income from one country in another after I moved, and those f*****s would happily tax it if I didn't realize my mistake. They did no checks at all about where the money is coming from, that they were already taxed and shouldn't be again due to the tax treaty. In fact I was not supposed to put it there at all. So I could keep my income from one country, and after move not declare it in the new country. And they wouldn't even know about it.
In fact they requested me to provide them proof that it comes from employment in the other country, when I demanded them to return me the money. So there was no information sharing between them at all.