It's because _forward is not technically a console command, it's a button, and so the _ is to indicate buttons for which there is a +/- state.
With the doButton / doImpulse commands, you can use: "doButton +_forward" and "doButton -_forward" So infact the +/- notation is not gone, it is just that you bind a key to the button, not to the up/down event.
I guess the fact that almost nobody knows the difference between buttons and commands is probably a good reason why they should mark the buttons differently.
The buttons have always been marked different. +button is the same as _button. I haven't really played Quake 4 or did any testing, but it seems pretty much the same to me.