My guess would be that, rightly or wrongly, most people just don't declare it at all. After all... "Bitcoin is aNoNyMoUs!"

Some countries have defined laws now regarding cryptocurrency and income and capital gains and how this should be declared etc... some don't. And I dare say that there is probably a significant proportion of people who don't even realise they are required to pay taxes on their crypto assets and income derived from such.

However, generally as far as the taxman is concerned... "Ignorance is not an excuse"