I believe this was intentional as many users complained about accidentally changing an important setting with their scroll wheels or trackpads. You can still click in the box and use the up/down arrow keys if you want.
There were a few complaints about it so i think they got rid of it. It was very easy to accidentally change settings. People complained of ruined prints from it. Its just one of those things i guess. You cant keep everyone happy. Just enter the values or use the up down arrows.
But if you have lets say 1000 users and 50 of them complain, do you really remove it then? The rest was probably happy.
I have never had any problems with it, just keep your fingers straight.
Unfortunately in the 3D printing world, if you make a few unintentional mistakes you can easily cause big damage to your printer. So I think it's much better to prevent these unintentional inputs.
And of course, you have similar functionality using arrow keys, but this ensures it is not a mistake
I would very much like to see this feature added back to the options. It probably would be best to have it as a disabled by default option, but the choice should be up to the user. Scaling is much quicker with a simple swipe of the scroll wheel but I can absolutely see how accidents can happen.