Disclaimer: This is not advice or an implication how to ride, but rather it's feedback to your inquiry.
The key to clutch-less shifting is timing and technique. There's a sweet spot (timing) when throttle is lifted/reduced, and it unloads the transmission (much the same as when the clutch is engaged). Because the clutch isn't engaged, the window of optimum time to shift (without clutch) is narrow.
That's where technique comes in. A nice confident/well-executed shift must take place within milliseconds, when the tranny is unloaded-- no lazy footing! Failure to do so can damage tranny components, if transmission begins to load before the shift is fully executed. It's not the sort of thing that a lot of people are willing to try or continue, in order to preserve the integrity of their equipment.
When I learned the technique, I rode with people who race and that's a common thing (clutch-less shift) in racing. When I witnessed it performed smoothly, it helps understand the "how to". If you've ever missed a gear when shifting(with a clutch), you've heard what negative feedback the a tyranny can communicate. That scenario can also happen when shifting without clutch, as well.
So the answer is: yes, it can hurt the tranny if timing/technique are flawed. If shifting is done flawlessly (timing/technique), it's not likely to damage tranny.
I've rode many of wheelies throughout the years (going through the gear box from first - fourth gear, on back wheel)-- never used a clutch. But, my attention level, alertness, and concentration were magnified by about 100 times, when doing this-- no room for error...hence timing/technique. In closing, and reiterating my disclaimer, I don't recommend or encourage clutch-less shifting.