Nothing? I've got a date format option for ISO date right now?

Where the fuck is that? Why can;t it just obey my locale which already produces the right format.

Right click on clock > "Configure Digital Clock..." > "Date Format:" > "ISO date"

Oh wow you can get the ISO date in one little corner of the screen but everywhere else a date is shown will still be mixed endian US.

You need to upgrade to KDE 11. KDE 10 reached end-of-life.