If your parents did a medical in the past 5 years and are currently in Canada and have not left Canada for more than 6 months, then they may have got the exemption under this new temporary policy.
Here is more info: https://www.canada.ca/en/immigratio...anada-requiring-immigration-medical-exam.html
Very unlikely to happen. Canada wants to ensure that applicants for PGP don’t have any “recent” health changes before they get PR. This is primarily for those who have had access to provincial healthcare system while they’ve been in Canada. They can see whether you have been using the healthcare system by looking at the billing records. More accurate than doing an immigration medical exam.