People with critically important privacy needs should never base their safety on the assumption that a Mastodon instance's data is secure from the feds.
If you share your data with ANYONE, you shouldn't assume it's safe from the Feds. The J6 people thought they were secure on Signal, but the Feds ended up with their chats, because the Feds has plants in the chats. And if anyone gets rolled, or their devices taken while things, like your data, are viewable, the Feds get that too.