Tell that to Sloan.
Federation News probably had a field day when it was found out that Section 31 infected The Founders and Odo is the one who put a stop to it.
For all we know, one of the conditions of the Peace Treaty might've been that it be made known what Section 31 did in the spirit of transparency.
Like COINTELPRO,
they were exposed. I don't doubt that something else probably replaced Section 31 afterwards, that no one knows anything about.
Section 31 in 2380 likely falls under "declassified". Although several of their actual activities probably aren't declassified. I don't know. I can't confirm or deny.