Looking up how to automatically lock your laptop screen when using the i3 window manager brought up a variety of methods. Several sites recommended creating a systemd unit to accomplish the task. A better solution was waiting in my default i3 config.

# xss-lock grabs a logind suspend inhibit lock and will use i3lock to lock the
# screen before suspend. Use loginctl lock-session to lock your screen.
exec --no-startup-id xss-lock --transfer-sleep-lock -- i3lock --nofork

There was already a solution in the config, it simply wasn't working by default as my Debian system did not have xss-lock installed.

sudo apt install xss-lock

Annoyingly, even after refreshing the i3 config, my screen would not lock. You can test this with xset dpms force suspend or by closing your laptop lid. I suppose you can use loginctl lock-session as recommended by the above comment as well. Sometimes, when things aren't quite working on Linux, logging out and back in again is a good troubleshooting step. Sure enough, this was enough to get i3lock working as expected whenever my laptop suspends.