Brad has submitted generic POSIX support to libuv for fallback support (I forget which platforms exactly were in consideration at the time). We even have a tracking issue for HP-UX support which tracks the status:
Part of it is HP saying they'd have a C++11 compiler and then not, some is modern GCC being a problem to compile as well.
C++11 and libuv have vastly improved the situation in CMake's codebase itself and letting HPE hold everyone else hostage for that would not have been worth it (and it takes a lot for us to drop platforms). IRIX was dropped because its owner killed it off and we're looking at dropping support for macOS 10.9 and older (in premade binaries; feel free to compile yourself) because we don't actually have tests running on those older releases.
https://gitlab.kitware.com/cmake/cmake/-/issues/17137
Part of it is HP saying they'd have a C++11 compiler and then not, some is modern GCC being a problem to compile as well.
C++11 and libuv have vastly improved the situation in CMake's codebase itself and letting HPE hold everyone else hostage for that would not have been worth it (and it takes a lot for us to drop platforms). IRIX was dropped because its owner killed it off and we're looking at dropping support for macOS 10.9 and older (in premade binaries; feel free to compile yourself) because we don't actually have tests running on those older releases.
https://gitlab.kitware.com/cmake/cmake/-/issues/20825
Full disclosure: CMake developer here.