Just got an email regarding this. It was apparently a system error:
Dear client,
You may have received a recent email from Immigration, Refugees, and Citizenship Canada (IRCC) with the subject line “We’re still working on your application” or “We updated your application status”.
This message was sent to some clients due to a technical error. If the message does not apply to you, for example if you no longer have an application in process with us, you may ignore it.
We apologize for any confusion this may have caused.
Sincerely,
Immigration, Refugees and Citizenship Canada