I went to System Preferences to set up the keyboard shortcuts ⌥⇥ and ⌥⇧⇥ for "Shift Right" and "Shift Left", respectively. The new shortcuts appeared in the Edit menu but neither of them work as intended and simply enter a space character instead.
(I'm working on the Finnish keyboard layout but the same thing happens when I switch to US. For my normal layout, the default key combinations for Shift Left/Right are pretty hard to reach, and ⌥⇥ is much more accessible.)