Does anyone know how I may delete some APP ID I've created into the Apple Provisioning portal and that are not used (created by mistake or for testing) ? I've read some answers from last year that says that it's not possible. Seems strange... And perhaps things have changed ?
Nope, last I checked you still can't delete App IDs.
IIRC, the reason given for this is that it's easy enough to simply create a new App ID for a new app, so why bother deleting old ones (or something like that).
Also, Apple seems to have a habit of making such things permanent and un-deletable (you can't delete Apple IDs, either).