Hello,
In OpenGL, you must enable the seamless cubemap filtering with glEnable(GL_TEXTURE_CUBE_MAP_SEAMLESS).
I never saw something similar on Direct3D but I read starting Direct3D 10 the cubemap is always seamless filtered.
So we had to use a trick to have seamless filtering to have proper lookup before Direct3D 10.
Is it ensured nowadays on absolutely all modern API (D3D12, Vulkan, Metal…) to be seamless filtering only?
Thanks!

It is guaranteed in D3D10, 11, and 12. I believe it's always on for Vulkan as well.