Sforno on Exodus 19:23/ספורנו על שמות י״ט:כ״ג

1וקדשתו, the term is used here as in Exodus 3,5 when Moses was warned not to step on “holy” ground with his sandals as this might prove fatal. Similarly, here; the people had been told that neither man nor beast would survive touching the mountain. [during this period. Ed.]