https://www.pcgamer.com/valve-doesnt-sound-too-happy-about-the-epic-store-copying-steam-data/

Yesterday, we learned that the Epic Store client was copying a Steam user data file called localconfig.vdf. According to Epic, the client stores a local, encrypted version of the file and does nothing with it unless you opt to import your Steam friends, at which point it sends only hashed friend IDs to Epic's servers.

Epic CEO Tim Sweeney said that this process was a rush job designed in "the early days of Fortnite," and that it's going to be fixed. He also said that the Epic Store doesn't use the Steam API due to Epic's own user privacy concerns, not regarding Steam's API in particular, but because of a "general concern of APIs collecting more data than expected." He pointed to this article about Facebook data harvesting.