Previously, a tool named the 'Steam Workshop Downloader' was used to download mods (This was discontinued as per Valve's request). These mods could then be manually installed into games which were not featured on steam (Xbox Game Pass, GOG, Epic Games, etc). The only alternative solution would be to use the Steam CMD console to fetch mods through an anonymous user / logged account. However, some games have this feature disabled, leading to said non-Steam games lacking any way to install mods from the Steam Workshop.

This is predominantly an issue as most mods are posted onto the Steam Workshop, which is the most popular distribution method for said mods. I understand (as stated in the Steam Subscriber Agreement), that Valve does not have any such obligation to distribute workshop content. However, there is currently no alternative solution to get mods for these games, leaving a large portion of the playerbase which would like to use mods, without any other option.

I suggest a way to verify that the user actually has a licensed copy of the game, no matter the originating store, and to provide mods for such users. If this seems a bit too difficult, steam could also just allow all users to get mods without requiring a game license.

Another final option (which seems like the best medium) is to allow mod creators to have a checkbox for this. This checkbox would make their mod able to be downloaded anonymously, letting anyone download the mod (without having to own the game on steam). This would give the creators their privacy over their mods as they want, while also accounting for other creators who wish for everyone to enjoy their mods.