This was the last official word from Zwift on how chat was changed in July…
As you can see they actually don’t define parameters by a specific number of nearest riders or a time radius, so it’s difficult to say how it’s supposed to work other than that it’s been purposefully throttled.
Edit: They say it’s limited to those you can see but it’s unclear of whether it’s those you can see on mini map or main screen. In any case, it doesn’t actually seem to work that way as I can often see chat from those I can’t see in mini map, on screen or on the riders nearby list. Overall it’s just inconsistent and hard to know what anyone’s talking about.