As a reference point, I’ve stubbornly been using A4 paper in both USA and Canada for years now with governments and businesses. Never had any problem with IRS, passport agency, CRA refusing to accept them, or even mentioning it at all in person.

No problem buying it either, though it’s about 2x the price of letter. (I go through about 100-200 sheets per year mostly due to my daughter drawing on it.)

I don’t actually store any printed paper since anything gets scanned and shredded, so no comment on long term filing.

(Due to this experience I recently partially switched to B5 paper, which is a nice size compromise instead of A5 which would be too small. Anything A4 will print to it without scaling issues of course, just takes up a bit less space.)