An approach for Blender 2.91 to track geometry changes of an object and update geometry of another object in edit mode. This could be used as a crude way to implement custom modifiers.

meshupdate.py

import bmesh
from bpy import app, data, types
from traceback import format_exc

handlers = app.handlers.depsgraph_update_post

handlers.clear()

# data.texts{'Output'] - stdout
# data.meshes['Source'] - source mesh (cage)
# data.meshes['Target'] - target mesh (modified source mesh)

def handle(scene, depsgraph):
    out = data.texts['Output']
    out.clear()
    try:
        target = data.meshes['Target']
        
        for upd in depsgraph.updates:
            if upd.is_updated_geometry and isinstance(upd.id, types.Mesh):
                # check whether we're interested in this mesh
                if upd.id.name == 'Source':
                    out.write(upd.id.name + '\n')
                    bm = bmesh.from_edit_mesh(upd.id).copy()
                    # APPLY MODIFICATIONS HERE (e.g. scale)
                    bm.transform(Matrix.Scale(2, 4))
                    bm.to_mesh(target)
    except:
        out.from_string(format_exc())

handlers.append(handle)