From the beginning of time, death was the penalty for not following God’s Law

Exodus 31:14-15

You shall keep the Sabbath therefore, for it is holy to you. Everyone who profanes it shall surely be put to death; for whoever does any work therein, that soul shall be cut off from among his people. Six days shall work be done, but on the seventh day is a Sabbath of solemn rest, holy to the LORD. Whoever does any work on the Sabbath day shall surely be put to death.