Different nations militaries have different philosophies, the only common thing seems to be the military is all about following their rules, and any benefit to the indifidual soldier has a lower priority than a benefit (real or perceived) to the mission.
What is, and isn't "authorized" varies with the time, place and people in charge you look at.
PERHAPS some militaries have now recognized that an individual carrying a handgun, in addition to their primary weapon isn't automatically the bad thing they previously thought it was, and are now issuing some troops handguns along with their primary arm.
I don't know. It would be nice for the troops, though. I do know that again, depending on who was in charge and how much of a strict "by the book and only by the book" commander they are has always made a difference.
Troops in combat generally have, and should have greater latitude than ones in garrison, and smart commanders will look the other way and not see things that are technical violations outside of combat areas.
The US has a long history of that, from WWII through Viet Nam, and probably afterwards, though I don't have personal experience with what they do today. During the wars, GIs in the front lines that "acquired" a personal handgun, technically in violation of the TO&E issue were generally not harassed about it while on the sharp end. I don't know about now, but in Viet Nam and before, personal, civilian sidearms were often sent to troops overseas, by friends or family, and they often were of great comfort and sometimes lifesaving use, despite not being authorized.
The general unwritten rule was, don't get caught with it during inspection, or when the brass comes around. I can tell you from personal experience that any pistol is a great comfort having when you're in your sleeping bag, or even in a foxhole, ALONG with your rifle. I know of more than a few cases where unauthorized pistols saved people's lives, and became "legacy" in units, where when the owner was rotated, wounded, transferred or otherwise left the unit, the pistol was left behind for someone else to use.
There is an old saying, with more than a degree of truth that, while a pistol won't win a battle, it might just save the life of the soldier that does win the battle.
IF various militaries are finally coming around to the idea that officially allowing a soldier to have a handgun as a personal protection arm, I think that's a good thing.
Ideas change, over time, and with experience, and while official approval of new ideas might be slow in coming, eventually good things do get recognized as useful. And, useless things eventually get discarded. Though that can take a while...
From the 1890s through the 1930s US service rifles featured a magazine cutoff. Good idea on paper, soldiers would fire individual aimed rounds, and the cutoff "saved" the rounds in the magazine for emergency use. Turned out to be a useless idea in combat, and was eventually dropped, after decades of it being a required feature. The brass is slow to learn some things but eventually, they do learn...usually....
In the Ukraine, I think its a matter of they are using (and allowing) everything they can get their hands on, no matter what the official TO&E actually is.