There's a PR for APNG: https://github.com/pnggroup/libpng/pull/706 – it seems there was some work for HDR in e.g. https://github.com/pnggroup/libpng/pull/635 as well. Related: https://github.com/pnggroup/libpng/issues/507
https://www.libpng.org/pub/png/libpng.html
Looks like this is the proper location for the project.
https://libpng.sourceforge.io/
