Modability is moreso a function of architecture than source code license, although being open source certainly doesn’t hurt.

Sure open source would not be a drawback but First place you’re looking for the information is in the documentation, not the source code. And then it depends on how the game is designed.

Sure worst case scenario an open source game you can rewrite the entire thing and document it as you see fit. But is it really modding then?

I find that question difficult.

I’d say the best modding experience is when there’s a documented API. You kind of don’t want to look at the code behind that documented API, because you don’t want to rely on behaviour that can change at any point.
So, if there is a modding API and it has all the features that you need for the mod you’re looking to create, then I’d say it doesn’t make a difference.

If there is no modding API, then as others already explained, you can fork said open-source project, i.e. you don’t even need to glue a mod at the side of it, you just change the game itself to work like you want it. In this scenario, open-source makes it significantly easier to modify a game.

I think open source eliminates one potential obstacle to modding, but having good interfaces is the main thing. If some open source code is unreadable spaghetti, then it’s gonna be hard to mod. Likewise if a closed-source game has well-documented interfaces available, it could be easier than a poorly-interfaced open source project.

I haven’t written video game mods but I’ve written a lot of code. Even in my own projects, ie where I have access and authority on all code, I want to be ignoring 99% of the code at any given time.

With good interfaces, you don’t need to look behind them. In that way, if the interfaces are good enough, it ceases to matter whether you can see the code behind them.

One thing you get with open-source games is families of variants, where the code for a game has been forked and developed in different directions by different developers.

OG roguelikes such as Nethack and Angband are an example of this. Rather than mods for a single game, there are variants — code forks that add new features, quests, monsters, skill systems, user interface elements, and so on. There are several dozen Angband variants. Some variants draw upon multiple other ones, combining patches to make a particular developer’s preferred form of the game.

Depends, was the game originally designed for mods?

OpenTTD has a ton of mods

Probably significantly harder. Open source tends to have a lot of churn and breaking changes. That’s more work to keep mods working. If the interface isn’t there for a mod, it could be added, but that’s more work for the modder.