This is seemingly a deliberate decision by management to funnel old players into sales. I stand corrected, it has been broken for like 4 years now, pretty much since the Classic Team had been laid off.
You must install the game every time using the new “Classic installer” after uninstallation or if you have installed the Battlenet app on a new PC.
You must have the RoC key registered to your account to play Reforged in SD graphics.
These classic installers are available from the Battlenet site somewhere, but this threads has the links: Classic Warcraft III --- Official Installers - Official Patches
If it’s the wrong thread, you’ll have to look around to find the correct “installer”. It’s only like 3MB, the download will continue through the Battlenet app.