Players in the WoW Classic demo have shown that sharding was implemented. At first a large portion of the community thought it was simply left overs from the client which is based upon version 7.3.5 from Legion. However in a blue post Community Manager Lore posted on the forums:
We’re still looking at how we can best deliver an authentic Classic experience at launch, and in the weeks and months that follow – both in terms of gameplay and community. You won’t see phasing (which is tied to specific quests that don’t exist in Classic) or cross-realm zones (which combine multiple realms together) in Classic. However, realm sharding is one of the best tools we have to keep realms stable when hundreds of players are swarming the same initial few zones and killing the same few mobs (like they will be at the launch of Classic). To that end, we do believe that some form of sharding may be helpful, especially in those early days. But we recognize that a cohesive world is critical to WoW Classic and are committed to bringing that to you.