International students whose fall 2020 courses will be online due to COVID-19 will not have their eligibility for a post-graduation work permit affected by online learning. Students in this situation may begin their classes while outside of Canada and complete up to 50% of their program via distance learning if they cannot travel to Canada sooner. They will also not have time deducted from the length of a future post-graduation work permit for studies completed outside of Canada, up to December 31st, 2020.
Source: https://www.canada.ca/en/immigration-refugees-citizenship/news/notices/pgwpp-rules-covid19.html