STIX 2.x documentation is available here. This site contains archived STIX 1.x documentation. STIX is now maintained by the OASIS CTI TC.
STIX 2.0 has moved. This site contains archived STIX 1.x documentation. Track the development of STIX 2.0 and TAXII 2.0 here. STIX is maintained by the OASIS CTI TC.