Risa

Spock flat-out said it at the end of "Such Sweet Sorrow, Part 2", albeit with a focus on the time travel shenanigans of the second season:

Regulation 157, Section 3 requires Starfleet officers to abstain from participating in historical events. Any residual trace or knowledge of Discovery’s data, or the time suit, offers a foothold for those who might not see how critical, how deeply critical, that directive is.

Therefore, to insure the Federation never finds itself facing the same danger, all officers remaining with knowledge of these events must be ordered never to speak of Discovery, its spore drive, or her crew again, under penalty of treason.