David kept his promise to Jonathan. On 2 Samuel 9, it narrates the story of how David showed kindness to Mephibosheth. He could have just sent money to Mephibosheth on a regular basis, but David did not just do that. He gave the very best to Mephibosheth, a self sustaining livelihood and a seat at the kings table.
1 Samuel 20:14-15
14 But show me unfailing kindness like the Lord’s kindness as long as I live, so that I may not be killed, 15 and do not ever cut off your kindness from my family—not even when the Lord has cut off every one of David’s enemies from the face of the earth.”
Does it still matter that David fulfilled his promise now that Jonathan is dead? I mean nobody would know. The answer is a resounding yes! It does matter. In fact God hates it, if we don't keep our vows and promises.
Numbers 30:2
If a man vows a vow to the Lord, or swears an oath to bind himself by a pledge, he shall not break his word. He shall do according to all that proceeds out of his mouth.
Now, if David, being human would keep his vow, how much more our ever loving and all powerful God. God would certainly keep his promise.
Joshua 23:14
“Now I am about to go the way of all the earth. You know with all your heart and soul that not one of all the good promises the Lord your God gave you has failed. Every promise has been fulfilled; not one has failed.