It was, but Oklahoma passed a law saying tribal lands couldn't use postal addresses and had to use PO boxes instead. Then they passed a law saying you needed a postal address to vote.
It was. And in fact the Supreme Court ruled in McGirt v. Oklahoma that the reservations still exist. One consequence is that Google Maps now shades the entire eastern half of the state in dark shading showing the borders between the Cherokee, Creek, Choctaw, Chickasaw, and so forth.
The tribal governments are taking an increasing role in providing public services to all of the citizens within their borders, as the civil state government descends deeper into libertarianism.