Perhaps 5% depending on the loco and its load, and the curvature of the track.
It also depends on how realistic you want it to look. Anything over 2% starts to look toy-like.

There are several main and branch line examples in- and outside Germany, old and new, with an incline of more than 2%. I guess they look toy-like.

That simple question and it has the usual answer of "it depends".
As others have pointed out, there are the optics that may come into play, but then the curves we can use are also unrealistic so its all a mater of taste.

From the practical perspective it still depends;
Which locomotive (power and # of traction tires), pulling what sort of weight, and how those cars behave (going up or down, do the buffers get entangled, couplers stay cleanly engaged).


So a single locomotive can do extreme inclines, put 3 passenger coaches after it and it cannot do as much, put a full 8-11 car train after it it does less, pull 30 or cars and its probably even less.
Run it up a straight with slow ease into the include (and out) and its one thing, in an R1 helix, its another thing.

Its on my "to do" list to test out the trains I'll be running by adjusting the elevation of the threaded rod helix I'm putting together to test the limits and know where I stand.

All I did was answer a question. I did not expect folks to talk about realism or aesthetics. I just like to run my Marklin trains while building this new layout. I like scenery and old German and Bavarian buildings (some modern also). So I guess my layout is a mixture of old and new. I really do not care. All I care about is how it looks to me and that I can have building it and running my trains.
My layout is 12 feet by 8 feet in an "L" configuration. Right now I am trying to automate it using signals as on my old layout that the late Tom Catheral designed for my. My 2 cents.

And what you like is the only thing that matters. (I assume you meant ...was ask a question...) So focusing on the mechanical physics of what can a particular locomotive do, pulling several cars - with Märklin it can probably go up a steeper incline than you would expect; but you want to test with your weakest (least traction and lightest weight, going around include R1 curves ) pulling your longest for it train, to find your limits. If the traction tires (rubber) are old, replace them first with new ones as old rubber gets brittle and less grippy often.

I'm inclined to agree!

I was waiting for someone to speak up, but then someone else might take a different angle on it - now that would ramp up discussion, particularly if another was to rise to it.

Really? IMHO, this is a progressive discussion.