How does removing combat XP actually help make the game less combat-focused and put more weight on non-combat solutions? It's just a band-aid to solve the problem of shit level and encounter design.
"We're gonna reward everyone equally by taking away the reward from the most commonly taken approach so everyone only gets the quest XP, and no additional XP from the way they solved it. This way we'll encourage people to attempt non-combat solutions!"
Uh, yeah. And yet the overall design still encourages the combat approach. LOL WUT
That's how PoE is designed. No combat XP because balance and encouraging non-combat solutions, but then you explore a dungeon and... there isn't really any other approach to it than combat. Yeah. Okay. Your game removes combat XP in order to encourage non-combat playstyles but then the dungeon design actually incentivises the combat approach because there isn't really a good stealth system to sneak past enemies, and there are plenty of trash mobs to fight. Okay. Totally consistent design there, yeah.
If you want to solve the problem of players hunting down a dozen trash mobs to get all the XP those trash mobs give, how about you solve it not by taking away the XP... but by taking away the trash mobs? Just some food for thought. Your players won't end up over-leveled if you don't supply them with a hundred trash mobs to grind through, and as a little bonus the game will feel less tedious to play through, too, because combat is less frequent but if it happens it's more exciting. And you get sweet XP for it. Everyone wins.
So, how would you go about balancing the rewards for combat and non-combat approaches? If the combat guy gets XP for fighting, the non-combat guys will be left out! So we have to remove combat XP to reward everyone equally!!!
No. Just give XP for non-combat solutions and everyone is happy. I'm playing through Atom RPG right now and you get XP for shitloads of stuff. Successfully pass a skillcheck? Get XP. Pick a lock? Get XP. Find out something interesting by talking to an NPC and picking the right options to get him to spill the beans? Get XP. Repair a device? Get XP. Etc.
If there's a dialogue/diplomacy solution for getting a group of bandits to leave the area, give the dialogue check a similar amount of XP as killing them all.
If there's a way to bypass a dungeon encounter by solving a puzzle that opens a secret area, give XP for solving the puzzle.
If there's a quest for an assassin to eliminate a target, give bonus XP for not being detected. Stealth games like Dishonored have "ghost" and "non-lethal" statistics at the end of the level that check whether you were ever detected and whether you killed any innocents. Just make the game check whether the player's character was ever detected during his mission and whether he killed anyone other than his target. Award a hefty XP bonus for not being detected and not killing anyone other than his target. Boom, you just massively rewarded a stealth character for playing stealthy. And it actually feels rewarding to pass the challenge of ghosting the quest, rather than the lame "we make stealth as rewarding as combat by taking away combat XP so fighters get the same reward as sneakers - none, lol"
But coming up with ways to reward non-combat approaches, and implementing these rewards, requires more effort and creativity than just stripping away combat XP to level the playing field. If no approach is rewarding, then all approaches are equally rewarding, aren't they? BALANCE!!!!1