Canadians have never been prevented from flying out of the country to any country that will let them enter. However, they must do a full 14-day isolation or quarantine upon return to Canada, even without any Covid-19 symptoms. The plan must be approved upon entry to Canada by a CBSA officer. These rules have been in place since the initial lockdown and invocation of the Quarantine Act in mid-March and have been extended several times.
Authorities have been monitoring those in quarantine for quite some time. My BIL has an exemption to travel for work to the USA periodically and each time he returns to Canada he has to quarantine for 14-days. Each time he is called and/or visited by local authorities (usually CBSA or the local police) at least three times. I have heard similar experiences from others I know.
Also, to make clear, currently only Canadian citizens, permanent residents, or those with a specific exemption, such as essential workers and certain family members, are allowed to enter Canada. Non-essential travel (e.g. tourism and recreation) is not one of those exemptions and no international travellers can enter Canada for reasons deemed non-essential.
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