Blocked tracking data of private operators

Is the FA blocking of private tail numbers required by law or just a service FA?

I was hoping FA would reply if this is a legal requirement or a service

Have a look on https://www.adsbexchange.com
I believe there is some information there.

It’s a mix; there are legal/contractual requirements to block some aircraft, and we also offer blocking as a free service to aircraft owners where they’re not covered by other block lists. (In practice, the FAA BARR / ASDI block list is used even outside the US). see https://flightaware.com/about/faq#blockmytail