The ULR-Key field worked fine in Kirby 2. Is it ignored in Kirby 3 or am I doing something wrong ?
The URL-Key field also works in Kirby 3, but only for the non-default languages.
Hi, Sonja 
well, I speak about the non-default language
(the default one is French or the URL-Key doesn’t work for English)
do I miss something ? 
What is different as compared to K2, is that you don’t define a field for the URL-Key. You can change the URL-Key for the non-default languages in the Settings menu, then choose Change Url.
(English is the non-default language here)
This will then add a slug field in the content file (its no longer called URL-Key)
Hi, Sonja
thanks to your screenshots, I guessed it would be enough to rename the ‘URL-Key:’ field to ‘Slug:’ field… in my text editor 
Is there a line about it in the Cookbook, Guide, Reference or How-to-migrate from K2 to K3 that I would have missed ? 
:-)***
Good question, there is a migration guide but its probably not complete.
Yes, you can do a search&replace in your text editor or do it programmatically.