You could use i3alter to add alt-tab (switching to other workspaces in a LIFO order) behaviour to your i3wm.
You could install it from PyPi:
sudo pip install i3alter
After running i3alter, hold alt and press <tab> or <shift>-<tab> to cycle between workspace.
You’d probably want i3alter to run on startup, and also you wouldn’t want alt+Tab and alt+Shift+Tab to be captured by other apps, so put the following lines in your i3 config file (/.config/i3/config):
bindsym mod1+Tab exec "true" bindsym mod1+Shift+Tab exec "true" exec_always i3alter