Are you asking about a 2020 or 2021 tax return next year?
Assuming 2020 and your divorce was not final until 2021: If you were still married in 2020 then how he filed is irrelevant to you. He could only file Married Separate if you did not sign a joint return. Just file your separate tax return.
Your question is confusing. As of December 31, 2020 were you married or divorced?
If you were still married as of December 31, 2020, you have to file as either married filing jointly or married filing separately for 2020. Your husband has the same two choices. For 2020, he is not your ex-husband if you were still married as of December 31. If you both file as married filing separately, you have to either both claim itemized deductions or both claim the standard deduction, so you each have to know what the other one is doing. To file as married filing jointly the two of you would have to cooperate to file one joint tax return together.
If your divorce was final on or before December 31, 2020, you would have to file as single for 2020 (unless you meet the requirements to file as head of household). You cannot file as married filing separately if you were already divorced by December 31.
The fact that the children are over 21 does not necessarily mean that your or your (ex-)husband could not still claim one or more of the children as dependents.