This character was code 0x01 in the original IBM PC code page (https://en.wikipedia.org/wiki/Code_page_437), and hence in DOS. It was displayed single-width and monochrome just like any other 8-bit character, never causing any rendering issues, unlike emojis today. It was added to Unicode for round-trip compatibility with that code page.

More background about how the smiley ended up in the IBM code page: https://www.vintagecomputing.com/index.php/archives/790/the-...