Having had a clutch failure in my early driving days, I can attest that one can change gears or disengage without use of the clutch pedal, but not without the use of the accelerator pedal.
The answer is in the difference of force being applied. When you are accelerating, you have a substantial amount of force being transmitted from the engine. In order to safely disengage, you have to step on the clutch.
When you are decelerating, the engine is slowing the vehicle, not as well as by braking and only the inertia of the vehicle (along with road grade) is applying force to the transmission.
If you were on a severe downgrade, traveling at a substantial speed and released the accelerator pedal, more force would be applied to the transmission. If your timing was not spot-on, there may be too much force to remove engagement.
Upshifting without a clutch is possible. Accelerate to the point at which a gear change is desired. Release the accelerator pedal, disengage the gear and move the stick to the next gear position. As you are doing this, the engine is slowing and will eventually reach the rpm required to match the transmission rpm, at which point the synchromesh will assist engagement and you have completed the shift.
For downshifting, the process is a bit more complex. Disengage while slowing in a similar manner, but while transiting to the next lower gear, it is necessary to blip the throttle to raise the engine rpm to better match the transmission rpm. The synchromesh will again assist, but timing is much more critical.
It's far easier to use the clutch. If you mistime your actions, gear teeth will be eroded/damaged, at least on the downshift. For the simple act of disengaging, it's less likely to cause damage.
