As per the wiki article someone else listed — the compose key was available on keyboards back in the 1980s (notably it was invented only 5 years after the Space Cadet keyboard was invented!).

Some DOS applications did have support for it. The reason it wasn't included is baffling, and it's especially baffling to me that other operating systems never adopted it, simply because

    compose a '

    alt-+
    1F600

    compose o c