I'm looking to try and create a reader to turn Magic Deck Files into decklists. But my first step is to work out where it stores those files.
You can find the profile information in
C:\Program Files (x86)\Steam\userdata\<userID>\316010\remote
your Steam path may be different, though.
Some info how to read the profile file can be found here.
Credit is due to christinemax from the Magic Duels subreddit - but I wanted to make this information available here as well.