I personally would consider Tolkien (the books in this case) as low fantasy, despite the epic scale and elves/dwarves. As well as the Witcher.
High fantasy would be something like D&D where you have bikini armors, flying castles and magic portals.
A lot of this has to do with the visual design of the world and its assets and the overall atmosphere. It all depends on how you implement it, not what you implement.
I’ve seen this come up a few times and I think the confusion is based on how the term low fantasy is used in games and how it is used when applied to books. People tend to mix these two when talking about games, so it can get really confusing what people are trying to say.
In videogames and tabletop role-playing games, low fantasy is used to denote a fantasy setting that tends towards the realistic. That’s it really. You then get into quibbling about what does and does not count as fantasy and what is and isn’t more or less realistic. To compare though, D&D would be high fantasy and the Thief games would be low fantasy.
It gets more complicated with literature, as there are multiple definitions you can use. You’ve got the one where everything that takes place in a different world is high-fantasy (generally with an epic scope) and everything that is in our world or a close equivalent is low-fantasy. Here Lord of the Rings is the quintessential example of high fantasy, while Harry Potter would actually be low fantasy. Another is that high fantasy is more about saving the world, while low-fantasy focuses more on personal journeys. So again Lord of the Rings again, compared to A Song of Ice and Fire. There’s way more ways of looking at high and low fantasy, but you’ve also got sword and sorcery, which puts the focus on the battles (Conan), or heroic fantasy which is about heroism and everything that that entails. Then there’s medieval fantasy, dark fantasy gothic fantasy, romantic fantasy and so and so on. Fantasy doesn’t get taken very seriously as an area of literary study, so you have to rummage around for definitions that sort of fit like this to get anywhere. It’s like how the only fantasy that is seen as having any literary merit is quickly declared to be magical realism.
As a game, Battle Brothers is probably closest to low-fantasy despite the fantastic elements, as most of them are treated in a relatively grounded manner. By the more literary definition and using Tolkien as another example, both take place in different worlds. Tolkien is about saving the world (good versus evil), while Battle Brothers is not (not in the good versus evil way, at least). Both are meant to be medieval. Keeping it simple, Tolkien would be high heroic medieval high fantasy, while Battle Brothers would probably be high medieval fantasy + sword and sorcery and some dark fantasy. This is why you can end up with some people thinking that it is a low-fantasy game, while others think it is high fantasy.