Boost is a nearly 30 year old open source library that provides stuff for C++ that most standard libraries for other languages already have out of the box. You seem to think that it is hipster bullshit rather than almost a dinosaur itself.
There might be, but as of a few years ago they were not mature and may not have captured the mindshare yet. Company I worked for actually used websocketpp because Boost ASIO implementation had some bug they couldn't work around, but then it was fixed and we dropped websocketpp.
I can say one the the nice thing about Boost network implementation (ASIO) is fairly mature asychronous framework using a variety of techniques. Also if you need HTTP or Websockets you can use Beast which is built on top of ASIO.
And if you're using one thing from Boost, its easy to just use everything else you need and that Boost provides to minimize dependencies.
The reason for depending on Boost in this repo is just few search characters away - he needs HTTP/WebSocket implementation and Boost.Beast provides it. The actual bloat here in this repo is conan.
For its credit though, it follows the C++ "philosophy" fairly faithfully. If you don't like Boost you probably don't like C++ either.
Although that download is a monster, I think its like 1.6 GB even compressed. Its not modular at all, some of the modules depend on others and its impossible to separate them out (they've tried in the past)
But last I check there is ALOT they could have removed, especially support for older compilers like MSVC 200x (!), pre C++ 11/older GNU compilers, etc. without compromising functionality. I'm not if they got around to doing that.
I see stuff like this, and I really have to wonder if people just write software with bloat for the sake of using a particular library.