It was not always ISO/IEC 20000. Considerable time and effort was invested by the IT Service Management Forum (itSMF) based in the United Kingdom. The itSMF provided the funding and support for the development and launch of the British Standards Institute (BSI) BSI 5000 in 2000. BS 15000 was based primarily upon the Information Technology Infrastructure Library or ITIL as it is more often referred to.
In 2005, following its success, BS15000 was superseded by what we refer today as ISO/IEC 20000, an international standard. Although the similarities could and still can be clearly seen between BS 15000 with its ITIL background and ISO/IEC 20000, the International Standards do not officially recognize any alignment to or with ITIL.
It is probably unlikely that the International Standards will reference ITIL directly as it is perceived as a UK or British Government initiative.
So as a word of advice for any organization that is considering becoming ISO/IEC 20000 certified and has not taken the ITIL route, it would be strongly recommended to take advice before going any further.