Since d20 past picks up about the time that firearms appear on the scene, there's no reason to change the soldier's firearms proficiency requirement.
Here's the AdC restrictions given for the three campaign models:
Age of Adventure: No Field Medic, no Gunslinger, and no Techie.
Shadow Stalkers: No Techie
Pulp Heroes: No Techie, No Field Scientist.
In general, the book doesn't change any requirements for past settings, though sometimes it swaps out benifits (Mostly for occupations).