John Gill Commentary 2 Samuel 19:23

"And the king said unto Shimei, Thou shalt not die. And the king sware unto him." — 2 Samuel 19:23 (ASV)

Therefore the king said to Shimei, you shall not die
This day by my hands, or order, or by the sword, (1 Kings 2:8);

and the king swore to him ;
that he should not die for that offence, or for that only; but if he committed a new one, this oath was no longer binding on him, and not at all upon his heir and successor.