If the work experience is in Canada, proof may include copies of T4 tax information slips and notices of assessment issued by the Canada Revenue Agency (the time period for these documents should reflect the work experience timeframe
It isn't a mandatory document, but it can help establish Canadian work experience. See IRCC policy below:
However, there is no obligation under the regulations that CEC applicants provide a T4 tax information slip or NOA specifically with their application, and these particular documents cannot be considered conclusive evidence or the only evidence accepted for the purposes of substantiating whether an applicant has qualifying Canadian work experience.
As such, in the absence of a T4 tax information slip or NOA, documents which may help substantiate the applicant’s work experience in Canada could include a record or letter of employment from the Canadian employer, work contracts and pay stubs.