Untested (i.e. just written from memory) and not really pretty either, but an alternative would be:
CGFloat deltaX = ((CGFloat(*)(id, SEL))[ event methodForSelector:@selector(scrollingDeltaX) ])(event, @selector(scrollingDeltaX));
CGFloat deltaY = ((CGFloat(*)(id, SEL))[ event methodForSelector:@selector(scrollingDeltaY) ])(event, @selector(scrollingDeltaY));

But conditional compilation isn't that bad either. I'd recommend breaking coding style here though to loose the second #if. Look at other OS X code for how.

I don't quite understand why the respondsToSelector-check is needed since you already check system version - but here's another way to do it which avoids runtime checks and may or may not be nicer:

/* Set the scroll count for scrollwheel scrolling */
#if (MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_7)
#define deltaSelector(AXIS) scrollingDelta##AXIS
#else
#define deltaSelector(AXIS) delta##AXIS
#endif
_cursor.h_wheel -= (int)([ event deltaSelector(X) ] / 5 * _settings_client.gui.scrollwheel_multiplier);
_cursor.v_wheel -= (int)([ event deltaSelector(Y) ] / 5 * _settings_client.gui.scrollwheel_multiplier);
#undef deltaSelector

respondsToSelector has three uses. First it allows the compiled app to run on any system version, second it avoids ugly #ifs in the code, and last the functionality of the compiled app doesn't depend on the SDK version it was build with.

Right - my question was regarding the original post. I wondered why it was needed when the compile time check is there anyway - my example omits that because if you have a compile time check there's no point in also doing a run-time check, unless I'm mistaken.

The compile time check/define itself doesn't prevent the app to run on an older system. Unless you stop that somewhere else, you still always need a runtime check or the user will get "unexplainable" errors.

Right you are - I foolishly assumed that everyone would compile on the computer they're playing on. 🤦‍♂️

Then another suggestion is to use @available instead, to skip #ifs and selector checks:

if (@available(macOS 10.7, *)) {
    _cursor.h_wheel -= (int)([ event scrollingDeltaX ] / 5 * _settings_client.gui.scrollwheel_multiplier);
    _cursor.v_wheel -= (int)([ event scrollingDeltaY ] / 5 * _settings_client.gui.scrollwheel_multiplier);
} else {
    _cursor.h_wheel -= (int)([ event deltaX ] * 5 * _settings_client.gui.scrollwheel_multiplier);
    _cursor.v_wheel -= (int)([ event deltaY ] * 5 * _settings_client.gui.scrollwheel_multiplier);
}

More info: https://clang.llvm.org/docs/LanguageExtensions.html#objective-c-available

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Completely forgot about this but thanks for the input! The function pointer suggestion works well, but with conditional compilation it's indeed not necessary (I got confused there).

@available looks nice! It was only introduced in 2017 though. That means that with @available any build on a macOS version below 10.12 would not be able to use the scrollingDeltaX/Y properties even though they were introduced in 10.7. There's probably not very many people doing that, but I'm not sure about the official stance on how well older platforms should be supported?

In any case, I'll get a PR in today using respondsToSelector.