A lot of stuff has to be reported at source these days, e.g. in Belgium where I live, Airbnb reports your income from hosting to the revenue service, your bank

A lot of stuff has to be reported at source these days, e.g. in Belgium where I live, Airbnb reports your income from hosting to the revenue service, your bank reports how much you paid on your mortgage (deductible), if you're unemployed then the benefits agency will communicate that, so will child care (deductible), and so on. And as the parent said, you can always add/edit amounts as necessary, which indeed will often be necessary for things like equity.