Disney Lorcana’s Fabled expansion marks not only the ninth set to be released for its lifespan. It also marks when Ravensburger is implementing a set rotation for the game as a whole.
Disney Lorcana Set Rotation Date

Ravensburger has made it clear that Set Rotation will start with the release of the Fabled expansion, which occurs on September 5th. That said, while it has been made clear that the Set Rotation will start with the release of the Fabled expansion, Ravensburger has always had two schedules when it comes to new sets.
First is a pre-release schedule, where the new product gets a head start to hit the shelves of local game stores. The second schedule, is the worldwide official release. For Fabled, the pre-release is scheduled to begin on August 29th. For the worldwide release, Ravensburger has advertised it to be on September 5th.
Despite prerelease happening first, it is widely considered that rotation will occur upon the official release of Fabled, which is September 5th.
Disney Lorcana Set Rotation Rules

Set Rotation is a system where specified cards from designated expansions/sets will no longer be legal for use when playing in official tournaments for Disney Lorcana’s main competitive format, Core Constructed.
Starting with the official release of Fabled, Ravensburger announced that Set Rotation will be updated annually every year. Currently, it is assumed to happen every August/September after Ravensburger begins a new cycle of four expansions.
Each update to Set Rotation will see the oldest four sets being removed from legal status in Core Constructed tournaments. For those not familiar with the release order of the Disney Lorcana expansions, the cards from the following sets will not be legally recognized in Core Constructed at Fabled’s release:

At the release of Disney Lorcana’s thirteenth set, the next four sets to be rotated out of legal status will include:
This pattern will continue indefinitely until Ravensburger announces otherwise. By this measure, players heavily invested in the competitive aspect of Disney Lorcana will have to tread carefully when buying different expansions. Make sure you're buying from Lorcana sets that are legal in Core Constructed if you want to play that format.
As a general note, cards with any special promotional versions (e.g. A Whole New World) still follow their original printings.
Disney Lorcana Set Rotation Reprint Clause

The Reprint Clause allows players to make use of any card that was originally released in a set no longer legal in the Core Constructed format if the card has been reprinted in a legally recognized expansion.
For example, Ariel - Singing Mermaid originally debuted back in the fourth expansion, Ursula’s Return. Normally, the Set Rotation would ensure that Singing Mermaid would not be legal/recognized for use in Core Constructed tournaments. But with Singing Mermaid's official reprint in Fabled, all copies of Singing Mermaid released in Ursula's Return will be recognized as tournament legal.
This is pretty common for any trading card game that uses rotation to mark legal sets. Magic: The Gathering employs a similar structure for its flagship Standard format, and commonly introduces popular reprints from years past to excite players.
As an additional note for this topic, Ravensburger has confirmed that the Enchanted rarity variants of cards will not be reprinted in any future set. With this in mind, collectors might want to hold onto any Enchanted variants they come across. This change could mean that Enchanted cards that have Core Constructed legal reprints could become incredibly expensive.