Yes, it's a feature of X. The full list is in `/usr/share/X11/locale/*/Compose`, and people who are interested in this should peruse and install <https://github.com/rrthomas/pointless-xcompose> or perhaps <https://github.com/kragen/xcompose>.

VT220 terminals used to have an explicit "compose character" key - afaik it was handled in the terminal firmware, so not necessarily an OS thing at all if we're being picky.

Upvoted for positive pedantry though :)