A few people on DS9 found out. I don't recall Sisko or Bashir "broadcasting to the Universe" afterwards - they went THROUGH CHANNELS - and nothing further really happened...more evidence that Section 31 is known at the very upper echelons of Starfleet Command, Starfleet Intelligence, and the Federation Council <--- And they see no reason to compromise Section 31 at a time they'll be needed against the Dominion (this WAS DS9 Season 6 where there was a Fed/Dominion War on.)
That still doesn't explain why Bashir, O'Brien and Sisko weren't taken to task for Sloan's death. To use your example, this would be like a military officer escaping court martial for kidnapping and torturing an NSA or Seal Team 6 agent.
At the very least, S31 people became a
lot more expendable in the 24th century than they were in the 23rd.
It still doesn't explain why Sloan uses "was" and "original" Starfleet charter when referring to Section 31's status, and not use the present tense.
Compare how Tyler and Leland talk, who still try to maintain some semblance of legality (note the fear in Leland about it being revealed he killed Burnham's parents), versus--
SLOAN: Hello, Chief. How's the family? Everyone okay at home?
O'BRIEN: What's that supposed to mean?
SLOAN: Nothing. I'd just hate to see anything happen to them.
BASHIR: Don't listen to him, Chief. He's just playing games with you.
SLOAN: That's easy for him to say. He doesn't have a wife and children to worry about. Trust me, Chief, if something were to happen to me
BASHIR: What? They'll be killed? I'm disappointed in you, Sloan. You don't usually wield such a blunt instrument.
Sloan shows no fear in threatening to kill O'Brien's family. Leland shows fear in being exposed for killing Burnham's. Simplest answer is often best--S31 is answerable to Federation in 23rd, no longer in 24th.