The following is quoted from the ITU website.
In the interests of DX news, DX-World has come to the conclusion this latest development is newsworthy and therefore published in full with relevant links. Equally, we are happy to make public any response from those behind the Z6 drive. Kosovo became a new DXCC entity earlier this year.
NOTE: It should be pointed out that the Z6 prefix appears on the ARRL ITU Table of Allocation of International Call Sign Series website yet it’s not showing on the official ITU Table of International Call signs.
“Following the inquiry of the Administration of Serbia of 16 March 2018 regarding the use of Call Sign Series Z6 by amateur radio stations of Kosovo*, I would like to provide the following statement:
Pursuant to Article 19 of the Radio Regulations, notably its provisions Nos. 19.28B and 19.33, the management of international series of call signs is a prerogative of ITU. Call sign series can be allocated only to the administrations of the ITU Member States by world radiocommunication conferences or, between radiocommunication conferences, by the ITU Secretary-General.
In this respect, the attention of the ITU administrations is drawn to the fact that ITU has not allocated call sign series Z6 to any of its Member States.Consequently, the utilization of call signs series Z6 by any entity without a formal allocation and consent of the ITU represents an unauthorized and illegal usage of this international numbering resource.
* This designation is without prejudice to position on status, and is in line with UNSCR 1244 and the ICJ opinion on Kosovo declaration of independence”.