I keep getting the same question, and it involves going to a one-lens strategy with the D850 and the 28-300mm f/3.5-5.6 lens. One reason why I keep getting that question is that a famous post processing person keeps saying "that's all you need." Recently the advice has added "just add sharpening to fix any blur."
First off, sharpening does not fix blur, unless you happen to have one of the exotic (and slow) deconvolution sharpeners, and even then I'm not sure I'd use the word "fix." When you sharpen, you alter pixel values to add contrast at boundaries, basically. That's not the same thing as optical acuity.
But let's get back to the main topic, which is whether the 28-300mm belongs on a D850.
The 28-300mm isn't a terrible lens. It's big and it's heavy, though. That sort of is the antithesis of "travel lens." The real questions, though, are two: is it really 300mm, and how is it optically?
Optically, it has strong vignetting, one of the worst bokeh's I've seen, produces a lot of flare (both veiling and specific), has mustache distortion at the short end, has a lot of chromatic aberration, and isn't particularly sharp in the corners wide open. Center sharpness is decent, but the corners never come close, even stopping down.
More importantly, though, is the focal length breathing. At shorter focus distances, the actual focal length of the telephoto end starts to drop down, and eventually reaches 150mm! You need a much more distant subject to get anything near the 300mm (e.g. at 150 feet the lens is a respectable 270mm).
So, if you want to use this lens as your "travel lens" with a D850, you're at 40.5 ounces (1148g), you'll be cropping some to get rid of the extreme corner issues, you'll need to be using a lens hood always (and still watch your backlit situations), you'll be doing a lot of chromatic aberration and linear distortion correction, and yes, you'll be sharpening the image more than usual.
I understand the all-in-one-lens thing, but I'm not convinced that the long end of that is where you want to be concentrating. 28mm is a horizontal angle of view of 65°. Bumping up to 24mm at the wide end, you're at 75°, a significant and useful difference while traveling, particularly for indoor shots.
To me, the travel lens for a D850 is the 24-85mm f/3.5-4.5. If you also need long telephoto, then carry the 70-300mm AF-P or the 300mm f/4 PF with you in your backpack. You'll note in my mini review of the 24-85mm I write "not exceptional in any aspect, but it also has nothing that I would call a flaw, either." That's more of what you're looking for in a travel lens. It's also only 16 ounces (not 30).
A case can also be made for the 24-120mm f/4—the image above was made with a 24-120mm—if you need a bit more reach and don't mind a bit more weight (23.6 ounces). But to me, travel should not just mean convenience, but also non-fatiguing (as in "not heavy").
The thing is this: if you bought a D850, you bought the best camera available for about a recent three year period, and still nearly the best today. I'm not sure why you'd want to spend that much money to get those beautiful pixels and then smear them in any significant way. Other than the big and heavy 24-70mm f/2.8E, none of Nikon's mid-range or longer zooms truly do the D850's pixels justice.
Indeed, for travel, I'd rather just have the Z6 II with the 24-200mm lens. It's far better than the 28-300mm, doesn't focal length breath, gives me a wider view at the short end, and is pretty much exactly the kind of lens the 24mp Z5/Z6/Z6 II are made for.
Which brings me to the final conundrum. Many people buy the wrong thing. They buy the D850 because I and others say it's the best camera out there and they want the best camera. Then they go and compromise the camera with a lens that is going to reveal its weaknesses with those well-behaved pixels the D850 produces. That doesn't make sense to me.
If you're mostly using a camera for "travel" photography, the D850 probably wasn't the right choice in the first place.
Some of us are privileged enough to have multiple gear kits. I don't do travel photography with my D850. It's either the Z6 II or Z7 II with the 24-200mm, or more often these days, the even smaller Z50 with the two-DX-lens kit. That's just in the Nikon world. In the Canon, Fujifilm, Olympus, Panasonic, and Sony worlds I can think of excellent travel kits, too. But the D850 and 28-300mm would be far, far down my list of choices.
