I do it as well, and I agree it's a great strategy when the extension due date is the same as the 1st qtr estimate due date.  Making a 4/15/21 payment for 2020 and applying it to 2021 is the same as making a 4/15/21 payment for 2021.  I'm not convinced that making a 5/17/21 payment for 2020 and applying it to 2021 will count as a 4/15/21 payment for 2021.

I have yet to see an official announcement from the IRS detailing which "tax acts" are being extended.  They are presumably using the "disaster area" authority under 7508A which references a long list of "acts" that may be extended for combat troops per IRC 7508.

But then, my brain is mush and I can't seem to comprehend the meaning of 7508A(d) titled "Mandatory 60-day extension" and how that might interact with the 32-day extension we've been told via press release.  I'm sure the answer's in the Regs somewhere.  Or maybe politicians are just making this stuff up as they go without regard to actual laws.  I'm convinced I don't understand what the rules are when Congress feels it has to ask the IRS Commissioner for the extension of a deadline THAT WAS SET BY CONGRESS, and they feel they have to do this the same week that they passed a 242 page law which included changes to the Internal Revenue Code.