I'm not picking on Slack specifically here, but this is just healthy skepticism when dealing with cloud services all in all.

All bets are off with governments involved. There are lots of compelling reasons for secretly disregarding the already set privacy policy. Including but not limited to patriotism, anti-terrorism, child abuse, human rights, crime or flat out regime criticism, political reasons such as opposition.

Pick your providers carefully. You trust them more than you think.