Good question. The administration admitted to the existence of these waivers in May, but refused to disclose their contents to the OGE, which is unprecedented among recent administrations. This secrecy was roundly condemned throughout the federal government, and notably and publicly by George W Bush's former chief ethics lawyer [0].

Trump was eventually forced to back down and the waivers were released, revealing such swampy critters as Michael Catanzaro and Shahira Knight [1].

EDIT: So, "secret" in a more colloquial sense than the technical "classified" definition.

0 - http://www.businessinsider.com/trump-lobbyist-ban-cabinet-wa...

1 - https://www.nytimes.com/2017/05/31/us/politics/lobbyist-ethi...