Looks like its not coming in Dec 😲 

In order to improve your overall experience, we have decided to refocus our efforts on addressing key issues and bugs. Since the launch of the September Update, we've been taking a deeper look at our internal development roadmap, the way in which we utilize public betas, and the content of our next updates.

Here is a summary of our plans:

• From now and into next year (2020), the updates will focus almost exclusively on addressing recent and longstanding issues, including those reported in the Issue Tracker, ultimately making the game better.
• We are expecting to have a series of these updates, which will be rolled out (approximately) every three to four months.
• Each update will also include a public beta that will run prior to the update's release, with a dedicated time to address major issues that appear.
• The first of these updates will have its public beta in December, with an expectation that the launch will be early in 2020.

As a result of our updated plans, we have made the difficult decision to defer the release of Fleet Carriers. We are now planning for Fleet Carriers to be released in an update in the second quarter of 2020, rather than in December 2019 as previously stated. This will grant us additional time to refine the feature as well as focusing time on addressing existing reported issues. It will also ensure the state of the game is in a better position to introduce Fleet Carriers, that will provide Commanders even more opportunities to interact with the Milky Way. We understand that this delay will be disappointing for some players, but do know that this decision is one that we do not take lightly, and is made with the best interests of the community and game at heart.

Ultimately, we are confident that this will improve the Elite Dangerous experience for all our players.

Looks like this update isn't coming until June:

Frontier Developments announces plans for the upcoming addition of Fleet Carriers to Elite Dangerous, their space exploration and combat game. They say the delayed update will launch in June following an April open beta for PC gamers and a May open beta for PC and consoles. Before that they will hold a livestream on YouTube on April 2nd at 2:00 pm EDT to premier some gameplay and reveal more details.

This update is now available on steam.