They aren't deleted. You can click on the error message here and it will take you to the tweet.I see tweets from CBS News are still available, so either the Twitter Account got taken down, or the Tweeter deleted the Tweets posted.
However, I looked at the SpaceX Starlink thread and the SpaceX tweets that Ilya posted now show “unavailable“. Maybe Twitter broke their API that XenForo uses to include Twitter content in the thread.
Twitter = X
Just checked and the tweets are showing again. Don't know what's going on. I've seen this before a few times.