The "Portability" section in https://www.fltk.org/cmp.php#CS_CPP_PORTABILITY is a bit outdated because it does not mention Version 1.5.x.

Are the requirements/restrictions documented somewhere else or still open for discussion?

Also the allowed C++ version (C++11, C++14, C++17, C++20 or even C++23?) should be documented there for all FLTK versions that are still supported.