boost VS FetchBoostContent

We do not control boost.org, and putting this on a subdomain imputes an authority for decision-making we don't have. Building it on some temporary domains, then presenting it as a choice is the only approach compatible with Boost values.

Not at all. There is no "they", the Boost Libraries is just a collection of individual libraries that each have their own author or maintainer, usually unpaid (although the C++ Alliance has changed that somewhat). The only funding that "Boost" gets is from running the C++Now conference, and some of that pays for the hosting of boost.org.

Though the links within it seem to be to boost.org and therefore fail to be resolved. Well I can manually replace them with https://www.boostcpp.org/ like:

Oh wow, it behaves incorrectly...when I visit http://boost.org/ or https://... it shows spam on my side, whereas when I visit https://www.boost.org/ it works as expected.

If you wondered why Boost headers look like hell that's because once your library ends up being popular, you're kinda stuck supporting quirky compilers -- either yourself, or accepting patches for it.

c) If you are concerned that you won't be able to do that from CMake, like with FetchContent or something, then there are scripts that can do that for you. Or you can use boostdep to bundle the subset of boost you need with your library.

You can fetch it with only the individual header-only modules on which it depends with https://github.com/alandefreitas/FetchBoostContent