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 '

is VASTLY more user friendly to type than:

    alt-+
    1F600

which I have met some windows users who memorize that combo for things like the copyright symbol (which is simply:)

    compose o c

It’s not mapped to any key by default. A common choice is the right alt key.

I wrote a short guide about it last year: https://whynothugo.nl/journal/2024/07/12/typing-non-english-...

My personal preference is the capslock key. I'm not using it for anything anyway

In Vim it's Ctrl+K. ;)