I enjoy being able to see and interact with other people in a bunch of different contexts who are like minded without being so similar it becomes an echo chamber.
I do miss all the world building sub's and projects on reddit, at this point I only keep my account to post in one community. Additionally r/dogelore and r/polandball are great, and it appears those subreddit are branching out to other platforms in response to reddit recent changes.
Pretty much impossible, especially with so many eyes on the project. It is possible to intentionally introduce vulnerabilities into open source code and use that as a backdoor but for projects like tor keeping that hidden for long periods of time is incredibly difficult due to the number of people independently auditing the code.