Because it didn’t do anything. Set it true. Nothing happened. Set it to false. Nothing happened. It was not used for one single thing.
What did you think it would do?
To use alpha… have alpha in the texture or have alpha in the diffuse colors or have alpha in the vertex colors… or all three… then make sure it’s in the transparent bucket and blend mode is alpha. For those last two, no amount of material parameter is going to set those for you. For the first, you’d have needed that anyway.
…but the bottom line is that useAlpha was removed because it didn’t do anything.