I stumbled upon this article, written by a tax attorney, in a respected tax magazine https://www.thetaxadviser.com/...

It implies that the 3-yr window on amending your tax returns and claiming a refund starts on the day you actually filed your initial return and NOT on the extended due date. Why does it matter? Because, as of today, Oct 9, 2021 you can still amend your 2017 tax return, provided you filed an extension for that return back in 2018. 

The article says - not necessarily! If your 2017 return was filed before Oct 9, 2018 - then the ship has sailed.

Puzzled, I started checking it. The article makes this statement based on an obscure 2020 memo issued by IRS attorneys for the IRS staff. I read the memo. The memo DOES indeed make this weird statement, and it does so as a matter-of-fact. As if it's common knowledge, which it is not. For support, it cites a specific court case from 1981.

From my quick review, I'm pretty sure that the IRS attorneys misinterpreted both the IRS statute and the court case, and I can see where they tripped. After the Oct 15 deadline, when I have more time, I will double-check my preliminary conclusion and, if confirmed, will reach out to the IRS counsels to fight their dangerous error.

For my fellow tax pros: let me know if you're interested in this fight. Privately, please. Details are too technical for this forum. For everyone else: stay tuned if interested in the outcome.