Here is an exerpt from CBSA website. If you are from the USA, there’s only 3 criteria. You can go no matter when your COPR was approved.
Did you travel yet?
- Essential reason for travel
- No symptoms for COVID-19
- Quarantine plan for 14 day
For outside the US, you have to be exempt for some reason, one of them being the March 18 rule.
https://www.cbsa-asfc.gc.ca/services/covid/non-canadians-canadiens-eng.html
Arriving from the United States
If you are a foreign national arriving from the United States, to enter Canada, you must prove to the CBSAthat you:
- are travelling for a non-discretionary (essential) purpose or are only transiting or are an immediate family member
- are not presenting signs or symptoms of COVID-19
- have a plan to quarantine for 14 days, unless exempted
These measures are currently in effect until June 21, 2020, and may be extended.
Arriving from a country other than the United States
If you are a foreign national arriving from a country other than the United States, to enter Canada, you must prove to the CBSA that you:
- are listed as being exempted from travel restrictions (including immediate family members)
- are travelling for a non-discretionary (essential) purpose
- are not presenting signs or symptoms of COVID-19
- have a plan to quarantine for 14 days, unless exempted
These measures are currently in effect until June 30, 2020, and may be extended.