> to use the square root character (√) as a checkmark ([filtered out])

IMHO that's an old-school-ism: I've never seen that in the post-Unicode world, but I recall its being common in the days of PC-compatibles, when square root was one of the ~256 characters you got, and checkmark wasn't, so you made do. In the same bucket with "using eszett (ß) as a lowercase beta (β)" or "using upper-half-of-integral (⌠) as a long S (ſ)." In fact Wikipedia's footnotes mention all three of these substitutions. (Or "using hyphen-minus (-) as a minus sign (−)." ;))

[1] - https://en.wikipedia.org/wiki/Code_page_437

[2] - https://knowware.dk/dos56/norton-commander-1.shtml — see the last two screenshots on this page, from Norton Commander

I’ll own up to being someone who does this when I’m on a Mac. The reason is simple: they don’t give you a way to type a “real” check. And though Emoji contain suitable ones, those aren’t always appropriate (or allowed, such as here).

On a Mac, the facility you’re given to pick emoji (which is also the same palette you’d need to pick a fancy checkmark Unicode) is horrifyingly bad - it’s slow and hard to use, and has been getting worse.

Of course, iOS, whose emoji picker is pretty nice by comparison, completely lacks a first-party character palette and doesn’t support the “option key” method, so on iOS, it’s emoji or nothing.