Also, while Unreal does not allow any way to access previous frame information in your shaders via nodes or HLSL, you do have access to the Velocity vector via post process materials.
If your issue is an animated mesh with different moving parts where you want to calculate velocity specifically per-vertex instead of just the mesh, you’ll probably have the most luck going with a baked vertex animation texture.
That being said, there is a PreviousFrameSwitch node. My understanding is that it’s very limited in uses and you can’t actually read previous frame data from it, but I would love someone to prove me wrong. Maybe something like this??
Another potential option is to potentially use a render target to store previous frame information. In your material, you could compare the render target to your current frame to get velocity.