YouTube asked the authors to report their income , otherwise the United States may start to charge up to 24% tax on their payments

As of June 2021, Google may begin to withhold US taxes on YouTube income of creators who do not live in America. Therefore, the company asks for tax information to be submitted to Adsense by May 31st

Based on this data, the company will determine if the revenue will be subject to US tax. If information is provided - the company will calculate the amount of tax, if not , they can withhold tax  from author's generated in all countries income at a rate of up to 24%.

This can be explained by changes in the US Tax Code. According to the US taxation system, YouTube revenues will be treated as royalties.