Blizzard updated the Warcraft 3 Reforged system requirements roughly a week ahead of release date but there isn't much reason to worry as the demands are quite low. Even outdated mid-tier components will perform well with the RTS remake.
Microsoft is with their games but other developers are still releasing titles compatible with Windows 7. This will be the case with Blizzard and Warcraft 3 Reforged as the game will run on Win7 and Win8 as well. Windows 10 is still recommended.
As for the minimum CPU requirements, an Intel i3-530 will suffice. AMD Athlon Phenom II X4 910 will suffice for Team Red but overall, both components are low end and outdated by today's standards. In other words, you are highly unlikely to have a CPU that's unable to run Warcraft 3 Reforged.
The story is the same with minimum GPU requirements as GeForce GTS 450 or AMD Radeon HD 5750 will be enough. For reference, GeForce GTX 650 leaves both of these in the dust and the 650 was a mid-tier GPU during its time. It was released eight years ago. Only 4GB RAM required rounds up the rather low minimum specs.
Recommended specs are also quite low since the mentioned Intel i5-6400 is outperformed by i7-3770, which is once again an eight-year-old component. AMD users will need a bit better CPU though as Ryzen 7 1700x is the recommended one.
On the side of GPUs, GeForce GTX 960 and AMD Radeon R9 280X are the listed cards, neither of which are considered even close to high-end hardware. 8GB RAM wraps up the recommended specs, meaning Warcraft 3 Reforged should run smoothly at max settings without emptying your wallet.