Now that Unity is gone, the usual way of setting a Compose Key (via Keyboard Preferences > Layout) no longer works (because in Ubuntu 18.04, Devices > Keyboard has no Layout tab).
How do we set a Compose Key in Ubuntu 18.04 Bionic Beaver?
You may use (GNOME) Tweaks to set the option.
First install Tweaks by running
sudo apt install gnome-tweaks
sudo apt install gnome-tweak-tool).
Alternatively, for those who prefer the GUI:
launch the Ubuntu Software application.
search for "gnome tweaks".
Then launch Tweaks and go to "Keyboard & Mouse" section. The "Compose Key" options is disabled by default. Click on the "Disabled" button.
Then a window will pop up. Toggle the switch at the top on and then you'll be select the compose key
To expand upon what @pomsky detailed in their answer, the official documentation covers the two solutions available.
One is through installing Gnome Tweak Tool via the terminal like so:
sudo apt install gnome-tweak-tool
And the other is typing the Unicode character's code point. To do this, press Ctrl+Shift+U, release all keys (you'll see an underlined
u), type the code point desired, and then press Space or Enter to complete.
For example, a common one I use is referred to as “Smart Quotes” which are U+201C & U+201D respectively. So you would type 201C or 201D after releasing the initial keys, and complete the process with Space or Enter.