The VCF/BCF specification is no longer maintained by the 1000 Genomes Project. The group leading the management and expansion of the format is the Global Alliance for Genomics and Health Data Working group file format team.
The main version of the specification can be found on https://github.com/samtools/hts-specs.
This is under continued development, please check the hts-specs page for the most recent specification.
A PDF of the v4.2 spec is http://samtools.github.io/hts-specs/VCFv4.2.pdf.
A PDF of the v4.1 spec is http://samtools.github.io/hts-specs/VCFv4.1.pdf.
VCFTools host a discussion list about the specification called vcf-spec.