Is Fluorine more toxic than Chlorine?
Fluorine is much more reactive than chlorine and would certainly cause more damage to living tissues. You can even check out https://www.youtube.com/watch?v=vtWp45Eewtw for some fun demonstrations of its oxidizing power too!
Likewise, compared to chlorine gas, I'd assume fluorine munitions would be significantly harder to manufacture, store, and handle for any side attempting to wield them in combat. (And, as Waylander mentioned in the comments, especially for &quot;WW1-era technology&quot; too!)
Also, due to its great reactivity, fluorine gas will readily react even with the atmospheric humidity to form hydrogen fluoride: $$\ce{2F2(g) + 2H2O(g) -&gt; 4HF(g) + O2(g)}$$ which, while still toxic, is much lighter than chlorine gas and so may not be as good at staying &quot;close to the ground&quot; and/or &quot;flowing down&quot; into an enemy's trenches as intended. Or even worse -- potentially thus dispersing even back to your own troops and damaging them as well!