My experience with self-moderated thread is very normal like any other thread. The only one that got remove or delete is usually off-topic or spam without any contribution to the thread. Most bounty thread today are just filled with weekly report posts so even if they are self-moderated, no one seriously uses it for any discussion.
locking the bounty thread should only be done when the project is completed or stopped. it will reduce the spammer to be too noisy in the thread. for the discussion thread, this key function is very important and the moderation of the OP must continue.