Couldn't Laurent release the previous method as an additional class or something?
He certainly could, but this is a bad idea. Users would get confused and would not know which method to use to get realtime input. The whole functionality formerly provided by sf::Input can also be achieved with sf::Mouse, sf::Keyboard and sf::Joystick. It is just this issue which is more complicated to achieve, in general the new input system is much cleaner.
I tried catching/losing focus but when I work with InputBoxes it still gives a lot of problems
Can you explain the problem more concretely? Maybe not here, I think you should open a new thread in the Window subforum