I use a keyboard layout (bépo, an ergonomic layout adapted to French) that uses modifier keys on Space key: AltGr + Space to type an underscore (Shift and Shift + AltGr are also used for special spaces).

But since my update to Plasma5, it does not work anymore: regardless the modifier keys, when I press the space bar, I always have a standard space.

Reproducible: Always

Steps to Reproduce:
1. Switch to a keyboard layout that uses modifier keys on Space key (like bépo)
2. Try to type an underscore (AltGr + Space)

Actual Results:  
The typed character is a standard space.

Expected Results:  
The typed character should be an underscore.

After searching a bit, I think this problem is that plasma manages the space bar differently. Anyway, I checked the configuration of the keyboard layout and it should work.

/usr/share/X11/xkb/symbols/fr:
[…]
xkb_symbols "bepo" {
    […]
    key <SPCE> { [space, nobreakspace, underscore, U202F ] }; //normal,   Shift,   AltGr,   Shit + AlgGr
    […]
};
[…]