Re: "no requirement of 'registration' for deactivated firearms"
If a firearm has been deactivated in that way, why the need to track it?
I can think of one reason, but I have no means of knowing if it is the reason.
It is impossible to know if any given firearm has been deactivated or not without a reasonably close inspection. As a result it would be all too easy for someone to carry out a robbery with a deactivated firearm without the victim knowing that the firearm was incapable of actually firing anything and thus not putting up any sort of resistance because of a perceived increased chance of being shot.
FWIW anyone caught using a deactivated firearm in such a way is treated as though the firearm was not deactivated so the penalty is likely to be quite or very severe.
"But officer it isn't a real firearm" simply won't wash; if the victim had reason to believe that a firearm was being pointed at them then the law proceeds on the basis that it was a real one.
Anyone brandishing a "deact" (or even a realistic plastic toy gun) in public runs a very real risk of being shot if the Police have reason to believe that there is an imminent threat to life, either theirs or anyone else's.