My Galaxy A6 has also lost its QR code reader - I don't frequently use it so can't say during which update it was vanished, nowhere to be seen.
I have tried re-booting but no success. There should be plenty of memory left for such processing requirements.
I might cynically suggest that it's been deliberately removed to encourage purchase of a newer 'phone - otherwise it works fine.
One could even say that it be illegal to remove a feature for such a reason: I can't see any other widgets that might have displaced it.
However, it may be that the QR code standard has been updated (although I can find no mention) and older hardware is incapable of coping with the new: if that's the case, then Samsung should publicise that fact.
