https://en.wiktionary.org/wiki/canary_in_a_coal_mine

> What do you mean with it, in this context?

The pouplar reason given for the anti-success taxation, is that its fair. Because they agreed to the social contract. But its highly disputed by those taxpayers. Such disputes are canary/early-signs of fair/subjective contracts. Catalonians vs Spaniards is the other.