I don't know why anyone would paste or type secrets into codepen which is public by default... But it always seemed like it auto-saved / sent your code over to the back-end, going back ten years roughly?
It's also a foolish thing to do. Any time anyone types anything into anywhere on any webpage, that data is as good as gone, it's out there. Back in the 90's I experimented with per-character logging on all form input boxes, just because I could. That's still possible today.
Yes, absolutely, there are good reasons for that. The point is that any secrets typed/pasted in there should be considered compromized
Edit: but then if you you quit (or crash) without saving the pen first there is no way to recover unsaved work regardless. That is, the pen must be saved manually at least once for the user to benefit from the autosave.
CodePen does not disclose this in neither ToS nor in Privacy Policy, only in Builds documentation they say: “As you work on CodePen, your Pens are constantly running through the CodePen Compiler”
Yes, that is for rendering
Edit: and as "kypro" pointed out also for saving the current work but if you you quit (or crash) without saving the pen there is no way to recover unsaved work regardless