The IRS shuts down the e-filing system at the end of every year for two months to perform updates and maintenance. Attempting to e-file the tax returns outside of the authorized dates will cause a rejection. Ensuring that the return has been accepted within a year is imperative because the IRS will take several years to send a letter informing you that they have not received it.
If a balance was due on the original return, that could mean additional penalties and fees since it could be considered late. A refund could be lost since there is a deadline for filing returns with refunds of three years.
The e-file closure date has not been given yet (as of 11/01/2021), but last year it was 11/18/2020 so at least make sure to file by that date.
Do you still need to file 2018-2020 tax returns? Contact me at firstname.lastname@example.org for instructions on submitting your documents.