This issue was triggered by discussion in #8813.
Apparently, MacOSX has deprecated daemon() a long time ago with the intent of moving to an OS-specific system for background services.
There's a similar case on Windows, where daemon() doesn't exist and never existed.

I'm not 100% sure that this is worth doing as on Windows and MacOSX actual users generally use the GUI, so won't bother with a headless bitcoind at all [for a long time we didn't even distribute a bitcoind binary for MacOSX]. But I suppose there could be a use case for bitcoind as a system background service even there and this would be the issue to refer to.

This could take the form of external scripts/wrappers, similar to that for Linux distributions in contrib/init.