Offer your thoughts or opinion on the input financial query or topic using your financial background.

I file 83(b) election, but did't include a copy of it in that year’s tax return
It matters because that is the requirement for the 83(b) selection to be valid. Since the context is 83(b) election, I assume you got stocks/options as compensation and didn't pay for them the FMV, thus it should have been included in your income for that year. If you didn't include the election letter - I can only guess that you also didn't include the income. Hence - you lost your election. If you did include the income and paid the tax accordingly, or if no tax was due (you actually paid the FMV), you may try amending the return and attaching the letter, but I'd suggest talking to a professional before doing it on your own. Make sure to keep a proof (USPS certified mailing receipt) of mailing the letter within the 30 days window.