Some will be, if C71 passes in its current state. Lots of "ifs" there.

If they do, any existing owners who have one registered before a magic cut off date will be eligible to be grandfathered into a new specific sub-category of prohibited firearms that will allow them to retain their existing prohib SAN firearm, and to acquire other firearms now categorised under the same sub-category of prohibs.

The history behind them (and this is glossing over a few details) is basically:

• A load got imported by Dealer X
• Dealer Y/someone else took umbrage for whatever silly personal selfish reason and whined that these firearms are not bespoke built upon semi-auto receivers but were repurposed select-fire receivers. They went crying to the RCMP and kicked up a stink.
• Almost literally overnight, the RCMP decided "built on a select fire receiver? Prohib" but this was further reaching than just specific models. Thus, rendering a load of otherwise law-abiding firearms owners technically criminals overnight, as they now possessed a prohibited firearm without the right permits.
• Gun owners made a lot of noise (with aid from other dealers, and groups) and the RCMP issued an amnesty for turning them in.
• Eventually the Harper-led CPC slapped the RCMP the fuck down over this given how C42 had passed and allowed them to do so. This happened before the amnesty was up so hopefully the majority of owners didn't have to turn in shit.
• This obviously didn't settle well with a number of people, so the Trudeau Liberals then decided to go tit-for-tat and reverse the reversal, with even further reaching legislative changes that would stop a future government from simply reversing this again, and instead requiring even more legislative changes and checks.

And this is pretty much where we're at. I'm sure there's more details and specifics, and my inferred motivations/reasons for why things were done are more nuanced than I've implied.