We are preparing an extension of Brotli into full blown shared dictionaries. The shared dictionaries include LZ77ish dictionary model (similar to those that have been supported by Brotli repository — but not RFC7932 use — and ZStdandard for a long time), but also the Brotli's (distance,length) pair mapping dictionary mechanism and Brotli's word transforms to get variations of the dictionary -- for example for plurals, articles, inflection, prepositions, and capitalization in human spoken languages. In addition to what we have in Brotli, we include a context model that allows the dictionary to be reordered based on the two last bytes in the stream.
There are a few other things in the scope of Shared Brotli than just shared dictionaries. It includes a large window extension of brotli, a patching mode that is faster and more dense than Bsdiff+Brotli, and a framing format.
Shared Brotli is described in https://datatracker.ietf.org/doc/dra...brotli-format/
In HTTP content encoding simulations of Shared Brotli we have often seen a 40–60 % reduction of data transfer in comparison to the traditional RFC7932 Brotli — after the shared dictionary has been transferred.