In our game Aftermath, we currently utilize push-to-talk as our default means of voice chat. A common scenario is players are in some third-party chat app with their friends throwing around gamer words. No one in Roblox can hear their speech, but if they have voice input enabled, Roblox will still moderate them.
This is a really bad user experience. If they say bad things, and Roblox players actually hear these things, then moderation makes sense. If it goes out in to the void, they should not be punished.
With this style of communication, players dont even want to take the time to click the microphone button. They would rather not talk to anyone than be moderated in a setting where no one can hear them (they have not pushed the push-to-talk key).