Dear All,
just a small hint: the official source of the NUTS codes is a regulation of the European Parliament and of the Council:
https://eur-lex.europa.eu/eli/reg/2003/1059
Thus, this is not a European Commission document so the Commission Decision of 12 December 2011 on the reuse of Commission documents, probably, does not apply to them. (The Commission itself does not have the right to stipulate the use of the documents of other EU Institutions.)

These data are, of course, repeated in several formats, so there are versions that belong to the European Commission:
https://data.europa.eu/data/datasets/estat-nuts-classification?locale=de
However, this seems explicitly open to use which is in line with Commission's general policy advocating fiercly open data (e.g. https://data.europa.eu/de)
Cheers
Gábor