Metal support will be part of Godot 4.0 using MoltenVK, as cybereality mentioned. I think that was mentioned early on in one of the Vulkan dev logs, but I cannot say with certainty as it was awhile ago. Regardless, I do know there are plans to support Metal in future versions of Godot, whether its Godot 4.0 or beyond.
C# will be fully supported as part of Godot 4.0, as mentioned in this development blog post. I thought it was already fully supported, but I guess not.
I do not believe there are plans to add Metal shader support to iOS currently. There will be a way to use GLSL shaders directly in Godot, and these will likely be converted to Metal via MoltenVK. This should mean its possible to get much of the benefit of Metal just not directly.
(Disclaimer: I am not a main Godot developer! This is all based on what I have read and heard over the past year about Godot 4.0, so please take it with a grain of salt)
