D20 is released under the open game license, so anyone can use or modify it.

License here: https://www.d20srd.org/ogl.htm

Basic rules, classes, monsters etc are included: https://www.d20srd.org/index.htm

I don't actually believe you can patent game mechanics. Specifically for D&D, there's nothing novel about rolling a polyhedron and adding the resulting value with some other static value on a piece of paper.

* copyrighted