A sale of a principal residence exemption essentially provides that any capital gains on the sale of the principal are tax-free. That, for many Canadians, can be one of the biggest investments that they have. It is also a significant tax expenditure. It is something that hadn't been previously reported, and so in order to ensure the government had information on who was claiming these exemptions and to help ensure compliance, it was added.
On June 10th, 2021. See this statement in context.