See, here's the thing: Riot is an F2P company. They will never implement in-client voice chat until one of two things happens:
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The first reason that would make them actually add it would be that it has a huge income potential, but as it's just voice chat (unless you buy it with RP), it has no inherent income potential, so it doesn't matter to Riot.
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The second thing that would cause them to add it would be if droves of people started leaving because of a lack of voice chat, but since that's not happening, this reason is also irrelevant.
So, since Riot is an ever so lovely F2P company who can't do anything but hump their wallets (as if BILLIONS a year isn't enough), you shouldn't expect anything to ever happen to this game that doesn't have some kind of income potential. Just a waste of time to keep asking for it, honestly. They will do the bare minimum to keep people paying for skins and other crap, and nothing more. That's just how F2P works.
Really do wish people never bought into the scams of F2P and DLC. Gaming would be so much better right now.