I mentioned this a bit the other day in #server-chat when someone brought it up, but I'll say it again here so it's more visible.
I've been looking at proximity-based voice chat for a few years now. I do think it'd be a fun addition for those who want to use it. Annoyingly though, it's relatively hard to do well. The one you linked Myst that's used on Hermitcraft is a client-side mod, which people would need to download to use. In general I try avoid features like that where client-side mods are required for it to work, as it makes it hard for everyone to experience the same stuff.
There is a feature in the Discord plugin we use for chat does have a voice module which is disabled by default which can offer something vaguely like proximity chat, although not in the way you described. A limitation of Discord means it can't do positional audio, or fade you out as you get further away. Instead the voice module just puts you in a group when you're close together, always at full volume, then disconnects you when you move away. If there's interest,
I would be willing to enable that as a trial so people can experiment with it, but I'd like to make it clear it doesn't function like you expect. I do keep checking the Discord API changelings though more frequently than I should, on the off chance they implement positional audio fully, and if they do I will jump at implementing proximity chat properly
That being said, I am looking at other methods of proximity-based voice chat. I've got a couple of ideas in mind as to potential solutions, but right now I don't know if they're even possible, or if they will function in a way I'm happy with. I reckon over half of the ideas I have die somewhere in the development stages because they're not up to the quality I like, I wouldn't be surprised if this meets the same fate, but it is definitely something I'm experimenting with.
