Where is the horizon line on straight on shot stock photos with white background?

Full body shots like this tend to be shot from around waist height - though that can change depending on the model & the intent.
So the 'simple' answer is it's between his waist & elbows, give or take. But that's not the whole story.

You can get a rough idea by dropping the model into some landscapes & seeing how it sits. You can see that [very approximately] getting the horizon around the model's waist tends towards it fitting the image; but only if you can place his feet in an appropriate position for his scale and only if the camera height & tilt approximately matches. Wrong height, or tilted too far up or down will quickly fail to look comfortable.
You need to push or pull the model close or far depending on how long a lens your landscape was shot too. The distance the model was shot from & therefore the lens length will also have an effect. From 2m or less [or worst of all, at arm's length on a mobile phone - big nose, little ears syndrome in extremis] you will see a greater difference in apparent sizes at extremities than if it was taken from 4m or more.
You also tend to have to put him slap bang in the centre so the left/right perspective matches.

As you can see, merely knowing where the horizon should be is only half the battle.

These were done very rapidly, some came out better than others. I've made no attempt to properly blend to landscapes or match lighting etc.

1. Close up. Need to pull model in tight. Feels like a longer lens on the landscape.

2. Middle distance. Shorter lens. Have to push him away to match.

3. On a slope, camera too high & aimed down… tough to get this one to work, as the angles don't match. You'd have to add perspective to the model [bad idea] This one just looks wrong no matter where you try to put him, or at what distance. Feels like you're looking up at him but down at the ground.

4. Camera tilted back slightly, need to crop knees to get away with it. If you attempt to pull the model closer, he appears to be standing in a ditch.

5. Camera too low - non-starter, can't fix it, unless you send him off so far into the distance you may as well not bother. The bench in the foreground plus the slightly raised path give you no chance at a match.

^{All landscapes taken from govt/council public information sites with no accreditation}

The picture allows looking up the subject’s nostrils and down at the top of their shoes, so the horizon line is definitely below the nose and above the feet.

The horizon line of a photograph will be at the height of the camera (though rise and fall camera movements can distort this relative to where the camera is aimed). With organic subjects like people exact determination will require more work. With rectilinear subjects it is easier to trace perspective lines.