Twitter is growing up and it has had to make some grownup decisions. This year, it started making it possible to remove problem tweets only in the countries where they were illegal. Twitter is an open platform, but it has to comply with laws everywhere in the world.
On the one hand, this means censoring hate speech, on the other it means dealing with copyright infringement. This isn't that much of a problem for Twitter as it can't host anything and people don't really use the site to share links to infringing material.
But because of Twitter's size, it does happen on occasion. Previously, when notified by copyright owners, Twitter would simply delete the offending tweet.
Now, Twitter has a new way of dealing with this, it replaces the tweet with a new one explaining that the original messages was removed because of a takedown notice.
The original message is still gone forever, but this new method of "withholding" the tweet is significantly more transparent.