Clarifying notes:
IRCC website gives notice that as of this fall, dual citizens with a visa-exempt passport must present a Canadian passport or special Canadian Travel Document when boarding flights headed to Canada.
At the border, at the PoE, citizens are expected to present Canadian passport. It is not illegal or really a problem to present a passport from another country (have done this myself, having forgotten to take my Canadian passport on one trip), but of course this may result in some further questions, a referral to secondary for verification of status, or perhaps even an admonition to have and use a valid Canadian passport when traveling abroad. Otherwise, though, citizens have a Charter right of entry into Canada, regardless what documents the citizen carries or does not carry. (Note: misrepresenting one's status at the PoE is another matter, and of course that too is a serious violation of law, not to be trifled with.)
Important Reminder: IRCC demands the surrender of the PR card upon taking the oath. Anyone who "lost" their PR card is not precluded from taking the oath, but if they later "find" their PR card, any attempt to use that could lead to real problems. This would be an overt violation of the law.
Anyone tempted to retain possession of an old PR card as memorabilia should think twice about this. It is not like keeping an old drivers' license (most provinces require the old license to be destroyed when a new one is received). Wrongful possession of documents like a PR card is far more serious. Keep a photocopy instead. Or at least use a hole punch or something to make it obviously not usable.
