Honestly, even if slings don't make it into the game in the end, the ammo mechanic simply has to change.
Being unable to shoot special arrows outside of using them as consumables a-la D:OS leads to being unable to use them with sneak attacks or other special moves (hamstring shot, horde breaker attack...). We really need a quiver slot even if special arrows remain rare-ish, otherwise their supposed utility remains heavily underused. It wouldn't even upset the balance all that much given how save DCs on them aren't that high, so even if you use arrows of thunder for every occasion you aren't going to dominate every fight by their grace alone.
It's not a D: OS thing. They were the same in Baldur's Gate, Baldur's Gate 2, Icewind Dale, Icewind Dale 2, Neverwinter Nights and Neverwinter Nights 2 off the top of my head, all games that predate D: OS.
