Recent milestones: C++20 published, C++23 underway
C++20 has been published, and work is now underway on C++23.
Starting in 2012, the committee has transitioned to a "decoupled" model where major pieces of work can progress independently from the Standard itself and be delivered in "feature branch" TSes. Vendors can choose to implement these, and the community can gain experience with the std::experimental version of each feature. This lets us learn and adjust each feature's design based on experience before it is cast in stone when merged into the "trunk" C++ Standard itself. In the meantime, the Standard can be delivered on a more regular cadence with smaller and more predictable batches of features. This approach also helps C++ compilers to track the Standard more closely and add both the experimental and the draft-final C++ features in a more consistent order. The current schedule is in paper P1000.
You can also visit this site to get the latest C++ standards committee papers. Reading through these proposals, you can track the C++ developing trend and know how does a cool idea turned into the standard. However, these papers ARE NOT and also SHOULD NOT BE TREATED AS the standard documents.