It was originally going to be an RTS for Windows and MacOS, yes, but development certainly didn't start in the 1980s because Bungie didn't exist until 1991. Its first public unveil, as an RTS, was at MacWorld in 1999. In 2000, Microsoft bought the studio and Halo
(the "Combat Evolved" subtitle was forced on Bungie by Microsoft, who was concerned that consumers would not look to a game titled Halo
for an FPS) became an Xbox launch title.