Minecraft wiki entry for Development_versions, associated text: Now displays the session type in the window title, such as singleplayer or multiplayer.
If applicable, the window title also specifies the type of multiplayer server the player is on, such as on a LAN or third-party server.
Example: "Minecraft 1.15.2 - Multiplayer (LAN)".
Available types: "Singleplayer", "Multiplayer (LAN)", "Multiplayer (Realms)", and "Multiplayer (3rd-party)".