I was curious about this given what happened with SDCH.
Here is what Wikipedia[0] says
> Due to the diffing results and the data being compressed with the same coding, SDCH dictionaries aged relatively quickly and compression density became quickly worse than with the usual non-dictionary compression such as GZip. This created extra effort in production to keep the dictionaries fresh and reduced its applicability. Modern dictionary coding such as Shared Brotli has a more effective solution for this that fixes the dictionary aging problem.
This new proposal uses Brotli.