Since BG3 is supposed to cap at level 10 and moonblades are legendary weapons, they shouldn’t be available. They are in the realm of vorpal weapons and holy avengers.
If BG3 gets an expansion that raises the cap to 20 or so, then sure why not?
But then again, Larian seems to love flashy things so perhaps we will see moonblades in this campaign. I wouldn’t be surprised at all at this point.
the Level 10 Cap is long overruled