This is the info from IRS (which makes no distinction between a divorce and a separation):
"If you're legally separated or divorced at the end of the year
You must file as single for that tax year unless you're eligible to file as head of household or you remarry by the end of the year."
(https://www.irs.gov/individuals/filing-taxes-after-divorce-or-separation#:~:text=If%20you're%20legal....)
