Roblox supports modding and has a "Developer Exchange" program [0] in which, from what I understand, there's an exchange rate for each microtransaction related to modders' content.

Steam Workshop [1] is a platform for distributing mods for games which support it on Steam. A few years back it introduced support for paid content [2], but was soon after removed due to community backlash [3].

[0] https://www.roblox.com/developer-exchange/help

[1] https://steamcommunity.com/workshop

[2] https://steamcommunity.com/workshop/aboutpaidcontent

[3] https://steamcommunity.com/games/SteamWorkshop/announcements...