I should use weird characters on my passwords. Especially weird characters that look like normal characters. Then I can learn off the compose key sequences and instead of a password manager I can write down all of the passwords on paper and laminate them and stick them to my monitor.
These work in both X and Wayland, even though they are an X feature. X/Wayland will consult the standard file, but will also consult a file ~/.XCompose. A number of people have created files with extended compose sequences, of which https://github.com/kragen/xcompose is possibly the best-known. If you install or change ~/.XCompose, you must restart your session (log out and back in again) for it to be recognized.
I used to bind the Compose key to Caps Lock, but I've recently decided that Right Alt is a better choice.
Another commenter mentioned WinCompose, which I have used very successfully with Windows 11. On a Mac, you might try https://github.com/Granitosaurus/macos-compose, which is quite usable, provided you install Karabiner Elements for the remapping.
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.
I will not actually do this