in the material properties, choose the transparency's behavior :
"opaque" means no transparency, that's why you don't have any changes in your viewport even if the shader you have configured is ok.
The "display settings" of the material is in the material panel, while the material itself is in the node editor.
There are 3 alpha modes :
"clip" is a straight "yes/no" mask, use it for very sharp mask like, well, like your leaves.
"blend" treats alpha mask with all it's shades of grey (usually more than 50, hahaha :-X)
"hashed" also treats shades of grey but using a smart "yes/no" pixels distribution, the result is average good and maybe lightest to calculate (and kind of super-stylish, why not !)