In Kate, if you type a NO-BREAK SPACE character, you can see it, it's in pale gray. But not in Konsole. 

The problem is if you're coding in PHP in Konsole and accidently type a nobreakspace character then PHP will throw errors. Even though you cannot see any error because character is invisible, just like a space.

My keyboard layout is French (Canada), Canadian Multilingual. I can print the character with ALT+SPACE with the right ALT only, it doesn't work with the left one.

This is the information of the character in xev:
xev output:
KeyPress event, serial 40, synthetic NO, window 0x4c00001,
    root 0x2a3, subw 0x0, time 310994640, (174,179), root:(176,202),
    state 0x90, keycode 65 (keysym 0xa0, nobreakspace), same_screen YES,
    XLookupString gives 2 bytes: (c2 a0) " "
    XmbLookupString gives 2 bytes: (c2 a0) " "
    XFilterEvent returns: False

And with xmodmap -pke, it is:
keycode  65 = space space space space nobreakspace NoSymbol nobreakspace

I can fix the problem with xmodmap by remapping ALT+SPACE like so:
xmodmap -pke > ~/.Xmodmap
nano ~/.Xmodmap
replace all nobreakspace with space

However I think Konsole should, just like Kate and LibreOffice, print the nobreakspace as a gray square character.

Reproducible: Always