AFAIK Roblox relies on a centralized server for the account system and you can't play without an account. You can't put up a Roblox server on your LAN to play with friends when the Internet is down. If Roblox servers are down, no one could play. Unlike Minecraft*, Terraria, Quake, IRC, etc. When you have a Place on Roblox it's like having a page on Geocities, where Geocities could still go down and take everyone's stuff with it.
*Minecraft might need a special client to dodge the authentication issue
Discord is also very much centralized, but a sibling comment already mentioned that.
*Minecraft might need a special client to dodge the authentication issue
Discord is also very much centralized, but a sibling comment already mentioned that.