Sharding appears to have been temporarily enabled on Moon Guard (RP realm), most likely due to the Running of the Gnomes event.
According to a recent Reddit report, sharding appears to have been enabled on Moon Guard, one of the biggest RP realms in the US region.
Reddit user lurkerlarry42069 posted two screenshots. The first one is a map of Stormwind showing players with an RP addon. The second one shows none of the players on the map are visible.
Apparently, this happens every year with the Running of the Gnomes event.
When too many players are in an area, the game automatically creates a new “shard,” a copy of that area. Players entering the area will be placed on the new shard. The world appears identical from the player’s perspective, with minor differences. Players can only see other players on the same shard. Think of it as a sub-server of the main server.