Of course, unless they call them (company) to verify the letter and the issuer of the letter tells IRCC that that letter is no longer valid for X reason.
Otherwise, yes you could get away with it.
However, if it is the same as your current employer and you are laid-off, you are required to inform IRCC about this. This might raise a red flag about the arranged employment. They could ask you for an updated letter or as I said, contact the person/company who issued the letter to find out if it still stands.
You could also not inform IRCC that you're no longer working for them, but I would not risk it.