The missing problem of motion vector for DrawMesh is not a bug or a regressoin but just have never been implemented in Builtin... This isn't related to SRP. So it is a feature request to add the support.
However there is an existing alternative, the RenderMesh API have been develop to currently fix all the issues with DrawMesh (which can be modify too much due to backward compatibility). See https://docs.unity3d.com/ScriptReference/Graphics.RenderMesh.html . RenderMesh support the motion vector