ITU officially confirms: Z6 prefix is illegal
In a repost by YT9TP (Serbia) it would appear that the International Telecommunication Union has officially confirmed use of the Z6 prefix is illegal.
This from ITU Operational Bulletin No. 1149, June 1st 2018.
Call sign series can be allocated only to the administrations of the ITU Member States by world radio communication conferences or, between
radio communication 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.