IIRC there are a few valid 5e weapons that don't appear in Early Access: slings, as you say, but also, eg, whips and ... I can't think of any flails? I suspect you may be right that the specific animations for these weapons might be why they weren't included originally. I'm also wondering whether for slings in particular there might have been some thinking to do about how they are handled, given they only take one hand to hold but as I understand it in 5e they require a free hand for loading so (unlike in the original BG games) couldn't be used with a shield per RAW.
Given I'm sure druids in particular would appreciate a ranged weapon, I hope we can look forward to slings in the full game. I guess it's possible that we might get some visibility of this at the Panel from Hell on 7 July, or we may just need to wait and see in August.
