Multi-Column for X collects no data.

• It makes no network requests of its own. It reads the timeline data your browser already downloads from x.com, in your browser, to lay it out in columns. Nothing is sent to the developer or anyone else.
• It has no analytics, telemetry, accounts or remote code.
• Your settings, muted words, and download history are stored only in your browser (storage.local).
• The Download button saves media from X's own servers (pbs.twimg.com, video.twimg.com) using your browser's downloads feature.
• "Copy diagnostics" only copies text to your clipboard (request names and counts, no post text, names or cookies). It is never sent anywhere.

The extension needs access to x.com and twitter.com to do its job, and the downloads permission only for the Download button.
Source code: https://github.com/Lynchalot/multi-column-for-x (there is no build step; what you read is what runs).