Regarding the ability to copyright the "method of playing" or "rules" of a game:

https://www.rpglibrary.org/articles/faqs/fl108.pdf

The US Copyright Office says it can apply to text or description, but not to the rules.

WoTC did have a patent issues for MtG, but I dont think that's going to work for D20 rules. Perhaps they will attempt to patent the OneD&D release?

Either way, it's quite clear that corporations will attempt to colonize and control our culture, and lie and scheme their way to expand their profit extraction.