I’d be remissed if we had a post about skywriting and not mention when comedian Kurt Braunohler hired one a few years ago:
The fact I did a lot of work in SNOBOL and in APL in my formative years gives my current C# a certain, shall we say, accent? It’s not exactly code smell, it’s more je ne sais quoi than that.
Broomwriting
How feasible would it be to skytype a functional QR code?
I’m guessing pretty hard, with extant solutions, and of course you’d need a calm day even if everything else worked
I"m picturing a system not unlike those old time scrolling marquees on the sides of buildings.
I think the smallest QR code is 21x21, so you’d need 21 planes, or else a way for the planes to make multiple passes and keep the new lines of dots aligned with the previous lines, which seems pretty hard.
That’s certainly how it looks from the ground. Although in ultra slow motion. I also don’t know how those scrolling marquees were implemented in the pre-computer days.
I’m pretty sure I read somewhere it was feeding punched tape through a machine. I’m trying to find a cite.
ETA: Looks like it was actually metal plates run past a sensor:
Not seen one in decades. Perhaps the writing is not recursive. 
And to answer the OP, I don’t remember seeing skywriting/typing since I was a kid and even that is indistinct. Of course, this excludes smoke used by air show exhibitions.
It would be a lot easiler with aircraft that didn’t generate wind or otherwise disturb the air as they fly past.
Can’t have been any harder than creating those player-piano rolls, or even better, the rolls for those multi-instrument band organs. And designing the program cards for Jacquard looms, which Wikipedia says was invented in 1804. How did they do those?
Jack Benny once did a skit in which he played Stephen Foster, author of much American folk music. In an argument with his wife, the wife grabs a shot-gun and tries to shoot him, missing, but peppering the wall with bullet holes. Foster fusses about this, then pulls the wallpaper off the wall, rolls it up, and sits it on top of the piano. Later, he picks up this roll by mistake and sticks it in the piano. And that’s how he composed the music to Camp Town Races.
ETA: In Sylmar (part of the San Fernando Valley) there is a private museum, one floor of which is dedicated to a collection of antique automatic music playing machines. One of them has four player-violins!