I looked it up, never would have thought it. Honda recommends this.
1st to 2nd 12 mph (20kph)
2nd to 3rd 19mph (30kph)
3rd to 4th 25mph (40kph)
4th to OD 31mph (50kph)
Sometimes I run mine down to 30mph in 5th but that is just easing along on flat ground. It will do it easy enough but I don't like to lug an engine like that.
I have seen charts like this for other bikes. I've never paid attention to them, because I believe that they are set up for conservative riding in order to maximize fuel mileage.
My first bike, a 1975 Yamaha 650 twins was rated for maximum fuel mileage at 31 mph in 5th gear. I tried it once. 31 mph was the slowest you could go in 5th gear, there for minimizing the wind resistance in that gear. But that's how the manual rated that bike.
If you shift that early, the bike has enough power to do it, but I doubt you are in your power band yet. It doesn't even start to get fun until you get over 3000 rpms.
So let me ask you this: Is he suggesting that you shouldn't run your engine over 2500? In top gear, you would only be going 50 mph at 2500. (I'm estimating based on my 1500). If you want to go 60 or 65, you have to be up over 3000 rpms.
No reason to limit yourself, unless you are seriously trying to conserve gas. It doesn't serve any other purpose to keep your rev's low.