EDIT: Seems dynamic music is back in style in some very recent games, many of which I haven't really played yet. Good. 
For me, it's dynamic music, the kind that some games had that adjusted moment by moment to what was happening in the game.
The best-known example of this in the 90s game TIE Fighter, where the moment more enemy (or allied) ships showed up the music would have a little additional flourish to acknowledge the shift in battle. There were pre-battle tension tracks, battle music, complications of battle, grandiose flourishes for the arrival of enemy or even allied capital ships, and victory and failure music all ready to flow into the next seconds of the game.
A lesser-known but still excellent example of this was in Ultima Underworld and its sequel, where drawing a weapon had its own special "preparing for battle" tension music, getting attacked had a jump-out-of-your-skin joltingly sudden musical start that actually scared me as a kid when I got ambushed, music for battles going well, going poorly, victory and defeat.
I wish more games did those sort of second by second musical changes, but they've sort of fallen out of fashion for the most part. 
Level builders/editors
Splitscreen
K/D ratio - I know it's still in a lot of games but quite a few mainstream shooters have removed it
Aim assist being in everything - if it's crossplay, sure, level the playing field, but if it's console v console there's no need for that shit.
yeah I know the reason but the solution should be to just have better moderation around harassment rather than taking away a competitive element
couldn't you just mute whoever was giving you grief?
The sacred number let's a player know if what they're doing is working.
It also gives a number for your friends to rib you for - if you've got a 5kd you're a sweaty nerd and if you're 0.5 then you're a trash tier noob. Just a bit of fun.
My k/d varies from 0.5-2 depending on the game, and that's fine. If someone says I'm shite at call of duty, well, they aren't wrong. If they're going to annoy me about it they get a mute. No problemo. 2 clicks and it's done. In a decade of playing COD I've never actually had to do that.
I've only ever received abuse in tryhard games like DOTA, and once again, mute and done. Why remove long standing features of games when you could just leave it up to the players to decide?
Yeah it was for player retention, but also a lot of the people they were retaining are probably kids that shouldn't be on the game in the first place. Player retention under capitalism isn't out of pride for the franchise or friendliness - it's to maximise profits. Look at franchise shooters these days - I'm not begging for a return to the 'piss filter' days of the 2000's but every game is marketed with a battlepass full of flashy camos and colours and epic wacky outfits because they explicitly want an audience of children (despite the game officially being 16+). Games feel infected by all of this hyperstimulating garbage (I know that's what gaming is in the first place, but it's worse than ever), and every game is more than ever chasing trends that Fortnite started. I know that's a tangent, but it feels like it comes under the same area.
It's not a serious issue - it's just preference. BF2042 was the first Battlefield to do it and I picked it up recently, and it just feels weird not being able to check how you're doing. Kills are wrapped up with assists at the end of game - but you can get an assist by knocking 1HP off a guy or possibly even just 'spotting' them. You can only see your OWN KDA in certain game modes. It's just strange. I'm not going to stop playing my favourite franchise because of it, but on top of all the other 2042 issues it just feels strange.
It only sounds like 'cod lobby veteran' shit if you read it that way - I said the complete opposite. I can't remember a single time I've had toxicity in over a decade of COD. Especially not gameplay - there isn't even really a way to grief in such an Arcadey shooter as COD - maybe in the new Warzone mode but I've never put hours into that.
I don't know why you're trying to paint me as some internet smuglord trash talker - back when I played dota I had thousands of commends for friendliness. In battlefield I sacrifice my KD to get revives and keep the team rolling. I like team stuff. It's why I'm bad at COD - it's much more of a solo game. I've always been a positive mental attitude player. I'm not letting people know they need to get better - I want that KD for my own personal information. If some guys gone 96/5, maybe I should go follow them for a round and see what they're getting up to. Would probably have a good time tearing up the map and supporting in their wake. I specified that the ribbing is reserved to your friends.
It seems like you've twisted what I'm saying to epicly PWN me for the hexpoints ratio.
tbh I think we should both get a life
ok
I think one of the reasons shooters have removed K/D is because, for most of them, having a high KDA doesn't necessarily mean you helped your team win. Depending on the game it might even be the opposite.
That's true, yeah. Battlefield plays so much nicer when the team's working together. Not just the gameplay, but the overall atmosphere.
I love aim assist. It just needs to be reframed as an accessibility option. Especially with gamepads, having some tracking and magnetism makes games where you have to aim so much more enjoyable. My wrists and thumbs just aren't doing the same things any more as when I was younger.
It's probably not easy to find the balance, but in any single player or casual multiplayer game it is a very welcome sight.
They're a lot harder to make with modern / 3D games.
But they also seem to boost a game's long-term audience by a lot, so it's weird they're not more common.
...I should make something that needs a level editor instead of always doing procedural stuff.
I'm no expert - what makes them harder with 3D?
From a layman's perspective isn't it just giving your users access to a bunch of assets - I suppose streamlining the creating process to be user friendly is the difficulty. I can make a custom Far Cry map no problem, but GMOD or Skyrim I was at a total loss with their creation software.
FarCry does a pretty good job. I don't mind if it's all a bit janky and you have to find creative workarounds to problems. GTA V seemed to also have a lot of good custom maps.
That's pretty much it, it's a push-pull between streamlining for sane folks and raw power/control for insane folks. Also the first one takes extra dev time since devs are insane by definition and so are already using the raw tools.