@greghendrix These are a good couple of questions. 😊 I'll try my best to answer them.
1. Yes, absolutely. Governments, for example. Last year, 23.9 thousand information requests affecting 54.9 thousand accounts were submitted, and Twitter handed out the information in ~38 % of the cases. They receive information requests from non-government folks as well. For example, during divorce proceedings, one party might be interested in what the other has been up to online.
Anyone who decides to compromise Twitter – as a whole or targeting individual accounts – may also be able to read DM's. Data breaches and leaks happen all the time. Depending on the vulnerability exploited, DM's won't be accessed in all cases. One from the top of my head that did involve DM's was the incident back in 2020. And the other incident in 2020. This year, the data leak affecting 5.4 million Twitter accounts involved personal data about anonymous accounts but no DM's.
Also worth noting is that "people that work at Twitter" is not static. You might decide that "hey, the current owner and the thousand of employees are 100 % good people and would never spy on behalf of a foreign country." Okay, that's fine. But what about the next owner and the employees of tomorrow? 😊
2. That's not a totally unrealistic scenario you're describing. Yes, the people owning your data can read it and decide what to do with it. For example, monetizing it, as Google famously did with Gmail until late 2017. (They read Gmail users' emails and displayed personalized ads based on the content.)
[B]ut is that different since Twitter is a company and the person running the game dev Mastadon server is just a person?
I don't know. I guess Twitter would have more money and resources to defend itself in court should it break any laws. 😊
When it comes to conversations online that are not end-to-end encrypted, I think about them as a chat with a friend in a public space, like a cafe or a park. You have reasonable privacy, but there's always a risk of someone overhearing your conversation.