This is what Rick Broida from CNET"s Cheapskate has to say about it:
https://www.cnet.com/how-to/how-to-download-the-original-starcraft-for-free/
Bonus deal: Free-game time! Late yesterday I shared the news that Blizzard is giving away the original StarCraft, one of the all-time great real-time strategy games. In fact, you get not only StarCraft, but also the Brood War expansion pack. Sure, the graphics are a bit dated, but the gameplay? Still awesome.
To get the freebie (available for Windows and Mac), all you have to do is follow the above link, then click the appropriate installer. Note that once you run it, the setup seems to include only Brood War, but don't worry -- you're getting the full game.