Specific example: for a while, color emoji worked on macOS emacs perfectly. Then it was decided that since linux couldn't support it, it needed to be disabled on macOS, or else people might want to use a non-free system. It was removed for many years.
Is there _any_ other example? For years now, this is the only one I've ever seen brought up to support this point.
On the other side, there are many MacOS-specific features supported by Emacs, with the recently added dictation support being one of them. If a MacOS feature is missing, it's much more likely to be due to a lack of manpower than a desire to maintain feature parity with Linux.
For the entire 10 years I was a Mac user, a Japanese professor named Mitsuharu maintained a fork of Emacs with MacOS-specific features. I never came across any explanation for why the Emacs project didn't accept his changes (i.e., why he needed to maintain his own fork) other than some maintainers' wanting to discourage the use of non-Free OSes.
The good news is that this fork was very competently maintained, so I had a good experience in Emacs on OS X.
The 2 or 3 times I tried "vanilla" Emacs (the one published on fsf.org) it had bugs that weren't present in Mitsuharu Emacs. One of those times vanilla Emacs did not work at all: it displayed a window, but the window was very small (i.e., incapable of displaying more than a few characters) and the usual ways of enlarging a windows on OS X had no effect. My (typing blindly) evalling (set-frame-parameter frame 'fullscreen 'maximized) also had no effect.
The homebrew package named "emacs" got you vanilla Emacs. To get Mitsuharu Emacs, you needed to install the homebrew package named "emacs-mac" instead.
Maybe vanilla Emacs got better on OS X during the past 4.5 years. I haven't had access to a Mac during that time.
I have my own story over here[0], and I’m pretty sure that there’s going to be a lot of instances similar to what I’ve experienced — blocked from merging or just fizzling out due to the lack of push on features that work on non-free OSes. There’s a reason why there are so many forks and patches for Emacs on macOS that are maintained for years that are not getting upstreamed.
[0]: https://news.ycombinator.com/item?id=44739359
This is the only one that has bitten me personally, but I probably only noticed it because it was working and then taken away. I just assume there are more instances where something was never implemented in the first place for a political reason.
Edit: I also remember hearing that for a long time an ffi was forbidden because someone could use it to call proprietary software. Don't have a source on that one though.