Despite espionage being enabled in the game, post-renaissance era there are no spies or diplomats able to be found. This is actually really annoying because my favorite win type in the game is the diplomacy win, but I can't trade AI Civs for world congress and there's no point in discovering certain technologies other than to simply get through them because reducing spy-stealing rate is redundant if there are no spies to be found. That being said, I do have some steam mods at work in singleplayer, but none of them even touch on espionage at all:
Artificial Unintelligence Lite
Reforestation
Promotions - Expansion Pack
Really Advanced Setup
Faster Aircraft Animations
Quick Turns
No Instaheal
InfoAddict
The Goths (BNW or GK)
Ethnic Units
Barbarians - Unlimited Exp
The Kingdom of Italy (Victor Emmanuel III)
Also, this is Version 1.0.3.279 if that helps
Artificial Unintelligence Lite
Reforestation
Promotions - Expansion Pack
Really Advanced Setup
Faster Aircraft Animations
Quick Turns
No Instaheal
InfoAddict
The Goths (BNW or GK)
Ethnic Units
Barbarians - Unlimited Exp
The Kingdom of Italy (Victor Emmanuel III)
Also, this is Version 1.0.3.279 if that helps