Because there hasn't been an issue about this, just chat in various places, a short summary:

We want to move closer to cygwin, in short, msys should no longer be a thing you target, just a slight variation of cygwin.

Done:

• libtool defaults to "msys-" prefixes even for cygwin, so we can build for cygwin libtool: override the soname_spec for cygwin as well #2779
• We are dropping various autotools build system patches by setting a cygwin target triplet, like db: drop msysize patch #3004, icu: drop msysize patches #2982 etc.
• Python now pretends to be running on cygwin python: pretend to be cygwin in more places #2496
• meson always only reported cygwin (initially patched, now inherited from Python)
• CMake was changed to pretend to be running under cygwin: cmake: Simplify MSYS support patch #2422
• Make uname pretend to run under cygwin via MSYSTEM=CYGWIN: uname: allow setting the system name to CYGWIN msys2-runtime#97

To do:

• Remove all msysize patches
• Change the default triplet to cygwin toolchain: replace x86_64-pc-msys with x86_64-pc-cygwin #5448
• Respect the CYGWIN env vars in the runtime as a fallback: Fall back to CYGWIN env var if MSYS isn't set msys2-runtime#70
• Change the DLL prefix from "msys-" to "cyg" (unclear if we should do this...??)

Overall Goals/Strategy:

• Share as much as possible with cygwin
• Make cygwin documentation apply to msys where possible
• In cases where possible and needed allow users to still detect msys in addition to cygwin