Fun fact: at the same time, the most famous C++ library, Boost, is reviewing the `async-mqtt5` implementation (https://github.com/mireo/async-mqtt5) to be included in Boost as Boost.MQTT: https://lists.boost.org/Archives/boost/2024/10/index.php
edit: boost.org is a blast from the past, still looks exactly like I remember it from like 2008. Down to the "Get Boost" shopped on an emergency off button!
Apart from that, Boost has quite a few goodies such as Boost.Json, Boost.Program_options, Boost.Interprocess, Boost.Lockfree, Boost.Unordered, Boost.Dynamic_Bitset, etc.
Some libraries, like Boost.Atomic, Boost.Thread, Boost.Chrono and Boost.Filesystem, simply became obsolete with modern C++ versions. In fact, they served as the blueprint for the corresponding C++ standard libraries.
Personally, I have been wary of using Boost in the past because it's such huge library, but CMake integration is actually quite good these days and I found it pretty easy to use. Documentation is also quite good IMO.