Post, retrieve and engage with Tweets

Tweets, Retweets, Quotetweets: A Twitter user can delete a status at any point in time. Deleted statuses cannot be reversed and are permanently deleted.  When the original Tweet is deleted, all Retweets are also deleted.  When the original quoted Tweet is deleted, the Quotetweet remains.

Protected accounts: A Twitter user can protect or unprotect their account at any time. Protected accounts require manual user approval of every person who is allowed to view their account's Tweets. For more information, see About Public and Protected Tweets.

Null Casted Tweets: Tweets created on through the Ads API are referred to as "null casted" Tweets and would indeed come through the firehose when they are created on the ads platform, either through the Ads API or at ads.twitter.com.
Sometimes in the original format payload you'll see below. If a Tweet has this, it was created on the ads platform, but not all promoted Tweets have this metadata.

  "scopes": {
  "followers": false
}