Anything in its own compilation unit would be isolated. Anything written using C++03 should compile without too much trouble in a more modern compiler.
Also threading is part of the C++ standard library.
I don't know what you mean by sharing dependencies on telemetry and logging. That sounds like adding questionable complexity to libraries that are simple and modular.
In isolation yes. But if it interacts you get events like the fact that gcc changed std::string between versions (from refcounted to one with small buffer optimisation for C++11 compliance) also introduction of move semantics and rvalue references can change interpretation of the same class declaration between compilers in different modes.
You seem to be talking about major changes to the fundamental set up of a project while not taking a comparably trivial amount of time recompiling compilation units.
How often are you changing major compiler versions and breaking standard library versions that you would have this expectation?
You've never linked to a binary? I have no idea what debian's Qt was build with or libssl or SDL etc were built with but we can link to them all the same.
> How often are you changing major compiler versions and breaking standard library versions that you would have this expectation?
You don't configure Jenkins to build against multiple compilers/versions of compiler? Isn't that super normal?
Just because you have five header files instead of five packages doesn't mean you can use all five headers at the same time. For example, some might require C++03 and some might need C++14 or newer.
Also, if all dependencies are single file with no non-standard dependencies, that means none share dependencies they should, like threading, telemetry, or logging libraries.