Topic on Talk:Psychonauts 2
On the Steam store page, under the minimum requirements, it says "DirectX: Version 11", so that should be enough to prove that it's officially supported (and also "OS: Windows 7 (64 bit)" which does not support DirectX 12 unless d3d12on7 is included, which it isn't).
I have also tried running it on a system that does not support DirectX 12 and it automatically ran in Direct3D 11 mode, without having to use -dx11. Trying to use -dx12 on that system resulted in an error message saying that DirectX 12 is not supported on my system and that I should run it without -dx12.