The W3C is raising money to fund further development of EPUBCheck, so that it can support EPUB 3.2. Please contribute if you can. Thank you!
On May 8, 2019, the EPUB 3 Community Group published EPUB 3.2.
On August 14, 2020, the EPUB 3 Working Group has been formed, which is now the maintainer of the EPUB 3 specification. For joining, participation, etc., please consult the group's home page, or contribute to this repository via issues and comments.
For any further help, contact Ivan Herman or Dave Cramer.