This wouldn't be a mandatory thing because people who don't want to install the mod can still play on the server. The mod in particlur is called Simply Voice Chat. It's a server side and client side mod that allows you to use a in game proximity and group chat function. I've seen servers use an alternative called plasmovoice as well. This can add a depth of immersion/a better way for players (who have it installed) to communicate.
https://www.curseforge.com/minecraft/mc-mods/simple-voice-chat