Your graphics card is not supported, as Blizzard has updated their API used to Metal.
The TL;dr portion is that your graphics card does not support Metal API.
The slightly longer explanation is that Blizzard has chosen to switch the API used to Metal. The requirements for this API is OS 10.13-10.15, and the graphics card must support it.
Unfortunately your card is one series short (7000 series among others) from actually being compatible with Metal API.
There are no other graphical API used as Blizzard is focused on more current gen systems when it comes to Mac.
To the other people in this thread, probably best you post your OS and graphics card if you’re experiencing similar issues.
System requirements at the following link.
https://us.forums.blizzard.com/en/warcraft3/t/warcraft-iii-reforged-system-specifications/