In previous versions of Maya users could copy weights weights from Mesh A to only the verts of Mesh B, if the Mesh B's verts were selected instead of the object. In 2024 you're met with the following:
// Error: Unable to get source geometry for skin weight copying operation.
However, the source and target skin clusters are properly listed in the copy weights dialog. Being able to copy weights to a sub-section of a complex model is a critical function.
To test this:
have two objects skinned to the same joints.
In object mode, select a series of verts that you want new weights on.
from the outliner select the mesh that contains the goal weights (this will deselect your original mesh)
right-click on the original mesh and select verts. Your previous verts should still be selected.
Run the copy skin weights command (with closest point on surface active).
The tool will error.