I'm not sure if the Linux Foundation is just a relevant example here or an actual contender to be the steward.
I kind of hope it's the latter. TTRPG rules are close enough to code right? Structured instructions executed by humans with some flavor text at least.
They really should just rename it to "The Foundation Foundation"
But regardless of that, open source licences have nothing to do with what the code is doing; they are about using certain specific code in other products. Code is not copyrightable, so you can take some closed-source code and replicate its functionality in a different way - using different language idioms, or a completely different language - completely legally.
It might not be patentable, except it is under some jurisdictions https://en.m.wikipedia.org/wiki/Software_patent