Valve responds to the recent issue:
We are looking into what information the Epic launcher collects from Steam.
The Steam Client locally saves data such as the list of games you own, your friends list and saved login tokens (similar to information stored in web browser cookies). This is private user data, stored on the user’s home machine and is not intended to be used by other programs or uploaded to any 3rd party service.
Interested users can find localconfig.vdf and other Steam configuration files in their Steam Client’s installation directory and open them in a text editor to see what data is contained in these files. They can also view all data related to their Steam account at: https://help.steampowered.com/en/accountdata.