From 27 October Elite Dangerous: Horizons will be available for all Elite Dangerous owners as part of the base game.
Everyone who purchased Horizons prior to 27 October will be receiving an exclusive Azure ship paint job compatible with ALL ships currently in the game as a thank you.
Frontier have posted on the forums (here) the following announcement:
We are happy to announce that Elite Dangerous: Horizons will become a free update for the base game on the 27 October 2020 for PC, PlayStation, and Xbox users.
After the fold down, anyone who owns Elite Dangerous will get access to Horizons content and features free of charge, and all future purchases of Elite Dangerous will include this content.
As a huge thank you to everyone who has purchased Elite Dangerous: Horizons over the past four years you will be receiving an exclusive Azure ship paint job compatible with ALL ships currently in the game.
Elite Dangerous: Horizons brings planetary landings to the Elite Dangerous galaxy, together with weapon crafting, ship-launched fighters and exhilarating multicrew co-op action. This premium expansion also allows players to put wheels on the ground and explore the galaxy up-close with the SRV Scarab ground vehicle, and access to many surface star ports and other locations
Elite Dangerous: Horizons will remain on sale on all platforms until 26 October to allow players a last chance to obtain the exclusive Horizon owner reward.