The Rgb9e5 texture format is widely supported and very useful for representing things like HDR textures in a space efficient format. It features 9 bit mantissas with no sign bits for each component, then a 5 bit shared exponent.

Vulkan: E5B9G9R9_UFLOAT_PACK32
Metal: rgb9e5Float
D3D: DXGI_FORMAT_R9G9B9E5_SHAREDEXP (might go back all the way to D3D9?)
OpenGL: RGB9_E5 (support is required in 4.6, and possibly has been for a while)

Seems completely supported under all apis, so should be clear to add. I would recommend the name Rgb9e5Float.