The Windows 10 1909 November setup license agreement has a provision which seems to suggest Microsoft must provide Canadians with the ability to entirely disable updates. Is there such a law?
There is such a law, full text here. Under the Anti-Spam law, you cannot install software on another person's device without express consent, if this is "in the course of commercial activity", see §8(1). This seems to have generated a small meme about computers no longer working, but not justifiably so. Section 8 lists cases where express consent is implicitly given (not really express concept, but it is considered to be given under the law), in particular when the software being installed is e.g. HTML, a cookie, or an operating system. The law does allow a person to withdraw consent, once given: see §7. However, the option that they describe is generally available and not limited to Canada. They probably explicitly say this because Canadian customers are likely to have some awareness of this law.