There is a path in the nearby woods that is very steep and I think can only be travers with a mountain bike or on foot. At the moment it is tagged as highway=footway. However it seems that the consensus is that footway is designated for pedestrians and usually has signage. Both is not true especially since you can use it with a mountain bike.
I think highway=path would be better, but I am worried that trekking bikes could be routed this way by some navigation service.
I guess I could use smoothness=very horrible to prevent this but I mean the problem is the incline.
I could also use incline but I don’t know the exact percentage and whether this information is even considered by navigation systems.
Thanks for your tips!
highway=path is definitely better for such trails than highway=footway. For additional tags for bicycle users have a look at
Key:mtb:scale - OpenStreetMap Wiki in general and
Key:mtb:scale - OpenStreetMap Wiki specially in view of the incline.
Edit: @whb mnanaged to squeeze himself in between … 
I agree with the other two answers, I would add that if you are in the US you might also add mtb:scale:imba as this is what most mountain bikers here understand (also use mtb:scale). Don’t forget bicycle=yes/designated as appropriate.
Thanks! I’ll do that.