It seems that vertices with zero weight will remain in the vgroup by default, and there appears to be no straightforward way to batch remove all zero-weight vertices from it except having to manually delete one by one from Vertex Paint panel from N sidebar.
So I wonder if there are some possible ways to batch remove them from the group. Since it does lead to unexpected results when using some modifiers.
(Or is there any operators can do something like "Select by Weight"?)