i honestly think i just forgot when i initially published it on itch.io, but my mystery TTRPG, FICSion, is published under a creative commons attribution 4.0 international license. This means you can copy and distribute the rules however you want and remix, transform, and adapt the rules however you want to whatever you want.
the rules i wrote a super simple and you don't need to just use them for mystery games if you don't want to. if you want to go write a ttrpg story and need some rules, use mine! if you want to publish your own set of add on rules to FICSion and even make money on it, go ahead! if you want to write your own mysteries to be played with FICSion, please do! there's no style guide you need to follow, no materials to add in about the license, just state that you're making stuff based on FICSion and link back to it and you're good!
a lot of ttrpg publishers that arent wotc are putting out new licenses and statements about all the stuff which is reminding me that i never fully applied the license to FICSion which is my own blunder because i defo did research the CC BY 4.0 license to make sure i wanted to self publish FICSion under it, i just. didn't fuckin do it. anyway, it's at least there now.