Ok, I've checked my own ETA status online:
Following an assessment of your file, it has been determined that you no longer meet the requirements for an Electronic Travel Authorization (eTA). As a result, your eTA has been cancelled.
Your file is now closed. Should you wish to reapply for an eTA, we encourage you to do so only once you have addressed the reason(s) for the cancellation of your application. At that time, you may submit a new application at www.canada.ca/eTA.
You are no longer considered to hold a valid eTA. As such, you should not plan or undertake any travel to Canada.
So I think you can say that as soon as you are PR, your ETA is cancelled and you cannot use it to travel. Which is logical.