First of all: I AM NOT A LAWYER I AM NOT A LAWYER I AM NOT A LAWYER

Some folks are not sure what to do in terms of copyright. One of the challenges of scenario-writing, instead of being a developer, is that you have to work within the confines of copyright and other people's games. If you can develop and then write for your own game, that's great, but many of us (me) are good at writing scenario but not good at the more mechanical side of development.

Here's a good rundown of what is and is not copyrightable in games, though they use the most famous and litigious example (the Dungeon Game): https://gsllcblog.com/2019/08/12/part1statblocks/

Personally, the game I plan on writing for has a community content program on DriveThru, but not here. Since I plan on using setting/lore concepts from the original game, I plan on publishing my Yojambo entry as a free ashcan version and then making a "nice" version for the community content program, at which point I'll remove the ashcan from itch. This is kind of a legal gray area, to be clear, but one I feel is probably not going to land me in hot water, any more than sharing free game content on a  blog would. 

If you want to keep yourself in the clear, you can always write a scenario for a game with a creative common license, like TROIKA! or Mörk Borg or, well, most of the other OSR games. (There's a reason there's a lot of scenario content for OSR games compared to other indies... it's easier to write scenario content when you know you're legally in the clear!) Other games have OGL or CC for their generic or SRD system (PbtA, FATE, FitD) but not necessarily for the specific properties you may be hoping to write scenario content for, like Blades in the Dark or Masks.

That said, ultimately I don't see Yojambo as an enterprise I'm personally doing to make money on scenario writing, I personally see it as a way to increase adoption of indies games among D&D fans by increasing the supply of scenario content, and as practice module writing.