Is it permissible to eat meat available in Canadian markets, given that the exact person who performed the slaughter is not known, and there are slaughterers from various religions (Muslims, Jews, Christians, Buddhists, atheists, Sikhs), in addition to Islamically slaughtered meat also being available?

A Muslim must be diligent and strive to eat what is lawful (halal (definition)). The slaughtered animals of pagans, such as atheists and Sikhs, are not permissible to eat. If a Muslim or a person of the Book slaughters an animal, then eat from it. If anyone else slaughters it, do not eat from it. If the slaughtering takes place in an abattoir that includes people of various religions, and you do not know who slaughtered the animal presented, then do not eat from it.