Is Slack one-on-one direct message private ?

For a company, it's essential to be able to monitor/review employee communications for legal/compliance reasons. That said, while you should assume that any communication made with your official email/slack/teams/whatever can be seen by the company if it needs to be (e.g. somebody sues for something, even something potentially unrelated to you, that creates a need to search for relevant records), it's unlikely that Slack is actively reporting your conversations to your boss.

As others have said, if you don't want your company to see something you're saying, don't say it at work or on their platforms. In the U.S. at least, you have no expectation of privacy at work. If you're worried about something you've already said, you might just be screwed. ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯