The completion progress and complete notification only reflect copying to the local cache and not when the data is actually written out to the removable storage.

I think this is a bigger deal than it sounds like when you consider cut and paste, or just moving files. The writing would've been thought to be done even if the cache is still flushing, so it deletes the source files, but the write can still fail after the source files are deleted. The device could fail, it could be unplugged, you might kick it by accident, or have a power outage.

what's next?

they'll allow you to have different wallpapers on different virtual desktops?

Fucking finally. Been my biggest gripe for about a decade.

Does anyone know if this fix will be backported?

Not sure if this addresses the same problem (please let me know if not), but a fix that I've used for the past couple of years is one suggested in these discussions:

https://gist.github.com/2E0PGS/f63544f8abe69acc5caaa54f56efe52f
http://lwn.net/Articles/572911/

essentially changing the values of vm.dirty_background_bytes and vm.dirty_bytes. File transfers with the USB have been smooth and fast, on Plasma/Kwin, with this fix.

And this post explains how to make such changes permanent:

https://unix.stackexchange.com/a/149140/515980

Maybe this is useful for people with older versions of Plasma.

I was just recently talking about this with a colleague, who in the end, just said "Windows is better".
Guess now KDE is better. Or has Windows fixed that problem too? (in that case it was kinda sporadic).

I'm gonna go out on a wild limb and guess that AI had a part in this.