If you separated or divorced in 2023, 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…
You can get their SIN number from your previous tax returns. If you’re not sure what their income is, you can enter $0 (zero) and say “NO” to the spousal amount.