I've never heard of any soldiers wearing diapers unless they were incontinent and needed the diapers for the incontinence.
However some soldiers might wear diapers just in case they cannot make it to the bathroom and need to relieve themselves.
But the majority of the soldiers do not wear any diapers and it would be hard for the soldiers to change the diapers when they get full enough to need changed.
It would be easier for the soldier to just go pee outside somewhere instead of using diapers.
But I have heard that some snipers who cannot leave their post and have to stand guard and cannot take their eyes off what they are watching have worn diapers to go pee and poop in if needed.
But usually the sniper or soldier will have someone else take their place while they go pee and poop so then diapers would not be needed.