Small nit, but my filenames (and Flow names) are complicated strings which tie to the asset pipeline (over 20 characters including a coded location) - so would like to paste the name in, but the save dialog doesn’t support much editing capability including paste. Would be a boon if that could be enabled there and any other such dialogs.
Unfortunately this isn’t easy for us to allow. It’s a limitation of the Qt framework.
Frameworks, the boon and bane of software engineering
Pasting works for me in the filename field of the Save file dialog on MacOS.
It works on Windows as well.
Don’t know. Just tried again multiple ways with different text in “Save As …”, command-V does not work, nor any of the usual suspects like command-X. Daniel’s reponse indicates it’s a known thing, so can’t explain that.
Is there a limit to the number of characters in a Macintosh file name? Could be that if you try to shoehorn in more than the limit, paste becomes unresponsive.
256 characters!
Works for me on two different Macs, running Mojave and Big Sur.
Also Cut and Copy.
Maybe check you haven’t changed the key commands?
Changed copy/paste key commands? Not likely! 
'tis the problems with frameworks. They’re great because they offload some major problem onto somebody else. But then you are stuck with, one, understanding how to use them and two, stuck with their issues. Maybe Qt has a bug where it works on some systems and not others, who knows? But especially widely used systems like Qt have many customers so things like this often sit around and fester basically forever because they’re not high priority.
To be fair, there are plenty of bugs within macOS’s own APIs (or indeed Windows’) that only manifest themselves for particular users with particular hardware and particular software setups. (And which fester for years.)
It’s worth doing a bit of troubleshooting, though. It would be good to get more data on who gets the bug, too.
Just a guess - do your “complicated strings” include some characters which officially are illegal in file names? (Note that illegal characters sometimes “work” until you do something that breaks them).