REG 310.9100-2 An automatic extension of 6 months from the due date of a return excluding extensions
for 2020 the due date 5/17/2021 therefore 6 months would be by 11/17/2021 (but maybe 11/15)
Notice 2021-21 consistently uses the word "postponed" and never "extended" to change the due date to 5/17. so my opinion is you have till the middle of November. otherwise, you would be out of time (normally 6 months would have ended on 10/15/2021).
however, you may want to contact the IRS Rep who authored the Notice
Jennifer Auchterlonie of the Office of Associate Chief Counsel, Procedure and Administration. For further information regarding this notice, you may call (202) 317-5436 (not a toll-free)
if you timely filed your return without withdrawing the excess, you may still make the withdrawal no later than six months after the due date of your tax return (normally October 15), excluding extensions. If you use this process, you just file an amended return with "Filed pursuant to Section 301.9100-2" written at the top. Report any related earnings for the year of the amended return and include an explanation of the withdrawal.
