You can still work after the expiry of the letter. But sometimes the employer doesn’t allow. Like my employer is asking me to submit work permit or extended wp ext letter. I have already raised the webform requesting them to give me the wp ext with extend date on it if In case the work permit is going to take more time.
https://www.canada.ca/en/immigration-refugees-citizenship/services/work-canada/permit/temporary/extend/after-apply.html
Why there’s an expiry date on the letter
Your letter automatically lists an expiry date that’s 120 days from the date we receive your application. The letter expires in 120 days because
it’s the service standard to process this type of application
many employers and health insurance providers will ask for proof that you’re allowed to work
an expiry date helps satisfy their concerns about your status and authorization to work in Canada
If we don’t finalize your application by the date on your letter
If we don’t finalize your application by the date on your letter, you can still keep working until we make a decision.
If your employer or others need proof that you can continue to work
you can show them this web page, or
if your employer is not satisfied with the information on this page, you can contact us