Under current quarantine requirements, is it legal to visit Canada for less than 14 days?

I just called the Canadian Gov't with this exact question. I need to enter Canada to pick up a non-essential item (a pet). I am planning on flying from the US to Vancouver, staying overnight and returning the next day. The hotline told me that departing early is absolutely permitted, as long as you exercise quarantine measures at all points while in Canada.

The number is listed here: https://health.canada.ca/en/public-health/corporate/contact-us.html

would it be allowable to visit for less than 14 days (remaining quarantined for the entire period) and then return immediately to where they came from [?]

In my (almost) personal experience: yes, it would.

I know someone who is in the opposite situation: a US citizen and a Canadian PR (living in Canada) who has been traveling back and forth across the land border a lot. They have several times spent fewer than 14 days in Canada between trips to the US. In keeping with quarantine, you need to exercise caution (see below) in traveling in both directions between your place of residence/quarantine and the US border.

The rules are:

• Ensure you have a suitable place of quarantine that has the necessities of life, is not a group living environment, and is not shared with those at risk of more severe disease.
• Go directly to your place of quarantine without delay and stay there for 14 days from the date you arrived in Canada.
• You must wear a suitable non-medical mask or face covering while in transit, unless you are alone in a private vehicle.
• Practise physical distancing at all times.
• Where possible, use only private transportation such as a private vehicle to reach your place of quarantine.
• Avoid contact with others while in transit:
  • Remain in the vehicle as much as possible;
  • If you need gas, pay at the pump;
  • If you need food, use a drive through;
  • If you need to use a rest area, put on your mask and be mindful of physical distancing and good hygiene practices.

The rules for discretionary travel as per website state that:

• Discretionary travel includes, but is not limited to, tourism, recreation and entertainment.
• If a traveller's entry is permitted, they'll be subject to mandatory quarantine for 14 days.

So it appears that it’s technically allowed to stay 14 days or less (or to be more precise it’s not explicitly forbidden to stay 14 days or less) but who would want to do this? As the Atlantic article linked by @MarkJonhson indicates, and a per anecdotal experience of people I know, there is a follow up by Canadian officials to “make sure” that people respect quarantine when they have to. There are even instances of Americans legally entering Canada fined for sightseeing stops on their way to Alaska.

Now, I imagine that if you plan to stay for less than 14 days, the officials at the border will need quite a bit more convincing about the seriousness of your quarantine plan than if you planned to say for an extended period. They might let you in or they might deny you entry on the grounds that this quarantine plan is not realistic.