Should we deprecate the one without version?

I'd write something like Only effective since version UnstableV2.

explicit, also take enum class by value.

I'll add a doc.
I don't want to waste time with the macro stuff given we know there's only one user.

I agree it's weird to take a const&, but it's what the others do

I can't make it explicit. It's not a constructor

You can't introduce another createXdgOutputManager() because it's not overloaded. You probably need to rename this method, e.g. createXdgOutputManager2.

Passing an enum as const& is wrong although it doesn't make much of a difference in practice.

You can't introduce another createXdgOutputManager() because it's not overloaded. He's adding an overload, I don't understand.

He's adding an overload, I don't understand.

Bad wording, my bad. We are allowed to overload only methods that are already overloaded.

From https://community.kde.org/Policies/Binary_Compatibility_Issues_With_C%2B%2B

You cannot...

For existing functions of any type:

add an overload (BC, but not SC: makes &func ambiguous), adding overloads to already overloaded functions is ok (any use of &func already needed a cast).

As you've pasted, it is BC, which is what frameworks promises.
It's something that we definitely do currently.

It's also SC for literally every conceivable invocation, and we know how the only user of this class is using it.

Please use KWAYLAND_ENABLE_DEPRECATED_SINCE to make the compiler bark whenever someone uses the deprecated version of createXdgOutputManager.