I seem to recall proximity chat was enabled for certain gametypes in Halo 3. It was great to hear the enemy talking nearby and think to yourself "Now, do I find and kill this idiot, or do I let him give away his team's strategy first?" It would be great to have this, even with friendly units, so that you only have to exchange words with other Guardians if you're in the same fireteam (with the normal chat settings), or if you're next to one another (with proximity chat).
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It was definitely in Halo 2 and I think it was in Halo 3 for a bit.