A year later, the FLQ kidnapped the Deputy Premiere of Quebec and a British diplomat. At the request of the mayor of Montreal, the government invoked the War Measures Act, declaring martial law and deploying the Canadian Armed Forces under the direction of the Quebec provincial police.
You could hardly pick a more dangerous time or place in Canadian history.
The Wikipedia page for a sibling comment points out that Montreal already had problems: bank robberies, riots and terrorist bombings.
So if it's a 'natural experiment' it's one against a background of high levels of disorder to begin with.
I think it's also somewhat of a caricature to imagine that a total police strike is somehow indicative of what a radical reduction on police activity would look like. Most people calling for "de-policing" would draw the line at "let's not send police after bank robberies, assaults, riots, home invasions, etc".