There's a big difference between a 22lr bullet going straight up and straight down again (as in standing directly underneath rook branchers and shooting up at them) and a 22lr bullet fired at the optimum angle for maximum range.
In the first instance it wouldn't be much more significant than a large hailstone if it fell on you. In the second case, it might kill someone a mile and a half away.
(That said, I won't be trying it anytime soon!).
Quick and dirty guestimate, based on military research from the 1920s:
Terminal velocity of .22LR bullet returning, having been shot vertically, perhaps 200 fps. For a 40 grain LR bullet that's 4 fpe, i.e. approaching full power air pistol levels, which would have to smart if it smacked you on the head or shoulders, maybe even seriously injure a child. I may be a bit out with the estimate, but I think it's in that ballpark. Actually I tried calculating the terminal velocity for a .22LR 40 grain bullet with CD of 0.19 theoretically and came up with a rather higher figure, but that doesn't take into account tumbling etc..
For .30 calibre the research suggested 300 fps terminal velocity. So say a 150 grain bullet would have 30 fpe, which is definitely dangerous, which was the conclusion. They were looking into this, being worried about risks to personnel.
And of course as soon as they are fired away from vertical, the risks become far higher.
I'm not convinced that every rook shoot was conducted vertically upwards. Some of it would surely have been done by shooting up into the rookeries at the "branchers" from a little distance, sometimes by large shooting parties, unlikely to be hitting a rook with every shot, perhaps getting through a lot of ammunition.
Some of the calibres were quite large, , say .30, 140 grains, 1500 fps. I'd expect one of those to fall vertically back to earth at much the same terminal velocity as a modern .30 calibre would, even though it was initially launched with much lower energy. That would be risky, vertical or not. The smaller calibres were comparable to a .22LR, so I'd also expect them to have had a similar lethal range if shot into the air at an angle. As for the lethal range of a large rook calibre, shot at an angle, with about four times the energy, that's surely got to be even more dangerous, over similarly long distances.
Perhaps they were just lucky in those days that if it ever happened it was a rare occurrence, and woundings not widely reported, whereas fatalities were, I've found several reports with just a little googling. Those certainly happened due to negligent discharges (or "accidents" as they used to be known), going out in large shooting parties can't have helped.
For things up in trees, surely that's the job for air rifles and shotguns nowadays ?