If you separated or divorced during the year, the Canada Revenue Agency (CRA) will still require your ex-spouse’s/ ex-partner’s information. The info is needed for a number of reasons, like eligibility for certain credits or deductions, handling property you still own together, etc…
Also, if you are separated, that is what you nee to put as your marital status. In the eyes of the CRA, you can’t ever be single again. You will be separated until you divorce, and then you will be divorced until you either re-marry or live common law with someone.