Lizzybet, travel restrictions is in place because of covid-19, & as a result, many persons are not allowed to enter Canada. However, those who have immediate family member, like their spouse in Canada, who have their visa already, whether before or after March 18, will be allowed to enter Canada, &
they do not need a written authorization letter from IRCC confirming they are exempt from travel restrictions before they can travel. Parents, children, common-law partner all fall in this category. Here's all you need to know about this, including what border officers would expect from you when you land before they can allow you to enter:
https://www.canada.ca/en/immigration-refugees-citizenship/services/coronavirus-covid19/visitors/immediate-family.html#how-citizens-prs
However, for the majority of others, including those with express entry visa, cannot travel to Canada, unless invited by the authorities, while some other categories need an authorization from IRCC before they can travel to Canada. Here's all you need to know about this:
https://www.canada.ca/en/immigration-refugees-citizenship/services/coronavirus-covid19/travel-restrictions-exemptions.html#foreign
So, in a nutshell, those with their spouses in Canada can travel, but the majority of others need authorization from IRCC before they can travel to Canada.