They want to make sure new feature proposals for C++ are being thought through more carefully:
> We see C++ in danger of losing coherency due to proposals based on differing and sometimes mutually contradictory design philosophies and differing stylistic tastes. For that reason, we recommend that you
(re)read [Winkel,2017] before proposing a new feature (language or library).
Here is the document they are referring to, Operating principles for evolving C++: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p055...
The way TDG articulates where things stand is sage-like. It's reassuring they're sympathetic to new language features impacting normal users, and not just the people on the group.