Use ctrl-shift-u followed by the unicode character code in gedit, firefox, gnome-terminal, etc. Leading 0s (after the ctrl-shift-u) can be omitted. Works in webapps like Evernote as well.

Unicode 13 defines 143K+ characters, so you’ll soon need a map: https://www.unicode.org/charts/

u+00bc = ¼

u+00bd = ½

u+00be = ¾

u+2153 = ⅓

u+2154 = ⅔

u+202c = PDF

u+202d = LRO

u+202e = RLO

u+00b0 = °

Roman Numerals:
u+2160 = Ⅰ

in sequence through

u+216f = Ⅿ