These folks at CountingWorks say it well:
Bottom line, there 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 carryover.
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.
https://www.taxcpe.com/blogs/news/preserving-capital-loss-carryovers
