Is there a date after which you're no longer allowed to file your 2020
federal or Connecticut state income tax return?
I can't speak to Connecticut, but for the IRS there is no hard limit on filing back-taxes. The IRS says you should file all un-filed returns. They can't audit returns older than 3 years unless there is gross understatement of income (6-year statute of limitations) or fraud (no limit).
You lose ability to claim a refund after 3 years from the filing deadline.
Regarding capital loss carryover IRS Publication 550 (page 66) states:
When you figure the amount of any capital loss carryover to the next
year, you must take the current year's allowable deduction into
account, whether or not you claimed it and whether or not you filed a
return for the current year.
This article from taxcpe.com on Preserving Capital Loss Carryovers concludes:
Bottom line, it appears there is no actual requirement to file a
return, where one is not otherwise required to be filed. However,
since the carryover is based upon the results of a prior year return,
the taxpayer would have to be able to reconstruct the prior year
return to prove the carryover if challenged on the amount of the
Best Practice: That leads us to believe that the best practice may be
to actually file a return and run the statute of limitations even
though it is not actually required.
I agree with that, just file now and avoid the hassle of gaps.