On the contrary, you may gain xp from objectives in D&D, no matter how many creatures you killed or not. There are many campaigns and GMs that use milestones for XP gain.
And even if you use xp per kill, you receive xp for quest competition and stuff like that.
Yeah as a DM and as a player, I effing HATE milestone leveling. It basically does the opposite; no reward for combat other than loot, and you ONLY level up based on plot progression. A very VERY bad system in a game where players can go off on 100 different sidequests and fights and get zero reward for doing so. At least with DnD 5e, I as a GM can be like, "yeah, you go up a level for x sidequest." That would not work well here.
For BG3, I think they need to keep the exp based level system, and just add leveling for objectives/non combat as well.
