I'm not the original commentor, but I tracked this claim back to the reporting of a paper by Steve Pomeroy, who tracked the number of housing units with rent under $750 CAD a month between 2011 and 2016.

So I expect the answer to your question to be "they are still available, but at a rent higher than $750".

They get bought by investment firms and such, then flipped for profits.

We have a real issue since the federal funding of affordable housing got cut by the federal government many years ago. Thus current owners/landlords are having issues keeping the prices affordable, then when they can't sustain the building anymore, they end up having to sell, and agencies with lots of money gets to scoop them up. So we are losing 15 affordable housing units for each one we build to real estates sharks.