The conversations that I recall from around 5-10 years ago draw a fuzzy line around this topic (which seems appropriate). On the one hand, Emacs (GNU, the FSF, etc.) don't want to encourage people to use non-free systems by making Emacs "better" on those systems, and on the other hand, there's a recognition that the "baseline system support" for various OS's varies, including across the free/non-free boundary. Combine that with the reality that volunteer software is built by developers scratching itches, and sometimes the non-free support will drift ahead for a bit.
For macos specifically, there's been an additional wrinkle in GNUstep, although that seems (from a fairly far remove, admittedly) to have become more and more vestigial over time.
All that said: there is support inside emacs for switching between light and dark modes, at least inside the (included) modus vivendi/operandi pair of themes. This includes the ability to set up modus so that it is configured for both light and dark modes, has a very simple switch between the two, and has a way to automate the switch (based on a timer and/or DBUS message, IIRC). In practical terms, I believe that it should be ok to develop this support for macos now as a "baseline system support" feature that is either present or very nearly present on Emacs regardless of platform. In fact, it would be best if the macos support for recognizing the system-level switch and hook into whatever modus uses.
Hope that helps,
~Chad