This stunt was achieved using a visual effect technique called a “glass matte" shot. The drop-off to the department store’s lower floors was painted on a pane of glass, placed in front of the camera and aligned with the real setting, creating a seamless illusion.
If you look closely, you will see Chaplin’s skate wheels disappear briefly behind the painting in one or two frames.
Screenshots:
Modern Times (Criterion Collection)
"A Bucket Of Water & A Glass Matte: Special Effects in Modern Times" (Criterion Collection)