https://www.polygon.com/blizzcon/2018/11/2/18057924/warcraft-3-reforged-classic-compatibility-blizzcon

Blizzard describes Warcraft 3: Reforged as a “complete reimagining” of its real-time strategy classic, which is probably the best way of explaining what it’s doing with the 16-year-old game. Part remaster, part rebalancing, part retcon, Reforged will be a major upgrade for the game that won’t leave players of the original behind.

During an interview at BlizzCon 2018, Blizzard’s Brian Souza and Pete Stillwell explained that players who own the original Warcraft 3 will be able to play against those who own Warcraft 3: Reforged, and vice versa. Maps and other community created content made in the original Warcraft 3 are also compatible with Reforged.