[quikee]

No. VP8 and VP9 bitstreams are frozen and stayed frozen when they decided to freeze it (they made that sure with automatic tests in libvpx that calculated the bitstream checksum) otherwise old VP8/VP9 files would stop playing. The difference is only with the approach as with VP8 and VP9 the "reference" decoder is the specification, which is not what many people like for various reasons (for example there were some errors in the implementation, which needed to be implemented by everyone because of that).

The first link you posted about VP8 just writes that they'll create an experimental branch (back in 2010) where incompatible bitstream changes can go in. Guess what eventually happened with those changes? VP9

The second link is the link to the unfinished VP9 specification, however with VP9 they still had that "reference" decoder code is the specification approach. As VP9 started to gain traction, the demand for the specification became greater, so they post-hoc started writing one but as you can see, they never finished it. Still this doesn't mean the bitstream isn't frozen.