日々適当

import bpy

scene = bpy.context.scene
materials = bpy.data.materials

for material in materials: 
    print("Material:", material.name)
    objsUseMat = []
    for obj in scene.objects:
        if obj.type == 'MESH':
                if len( obj.material_slots ) > 0:
                    if obj.material_slots[0].material == material:
                        objsUseMat.append( obj )
    
    if len( objsUseMat ) > 0:
        for obj in objsUseMat:
            print( obj.name )
            bpy.data.objects[ obj.name ].select_set(True)
            bpy.context.view_layer.objects.active = obj
        bpy.ops.object.join( )  
        bpy.ops.object.select_all(action='DESELECT')