Age of Empires II: Definitive Edition Save Game Location
Windows
Age of Empires II: Definitive Edition save games can be found on Windows here:
C:\Users\[user_name]\AppData\Local\Games\Age of Empires 2 DE\[user_id]\savegame\
Linux
Age of Empires II: Definitive Edition save games can be found on Linux (Proton) here:
~/.steam/steam/userdata/[user_id]/813780