The PnP D&D5 games I play certainly don't link XP to killing and looting.
My current GM, in fact, doesn't even bother with XP but simply levels us when he feels it appropriate. It works. I can't remember the last game of Pathfinder or D&D I played where we gathered XP simply for killing things, it has been linked to story and completing quest-lines for as long back as I can remember. That's the beauty of PnP RPGs, your GM/group can determine how such things are handled.
Thats lvling by threshold or atory progression. And thats indeed a viable way to approach leveling. It generally prevents murderhoboing for 1 and thats always a good thing to prevent.
In bg3 tho we do get xp for killing things and outside of completing quests and succeeding skill checks seems to be the only way to gain xp.
Some fights simply dont reward the player imo. Phase spider fight in particular is horrid. Nezt to no xp for a very hard fight and the material reward you do get is....quesionable considering what you had to fight to get it.
