Siege of Mandalore Development
The Siege of Mandalore was first mentioned in "The Lost Commanders",
the premiere episode of the Star Wars Rebels television series' second
season. The episode, which was written by Matt Michnovetz, aired on
October 14, 2015. Michnovetz included the mention of the event as a
reference to the unreleased series finale of Star Wars: The Clone
Wars, the television series that preceded Star Wars Rebels. The word
"siege" in the battle's name was first capitalized in DLC for the 2017
video game Star Wars Battlefront II.
Previously, as the head writer for The Clone Wars, Michnovetz detailed
the Siege of Mandalore for a story arc that would have concluded the
series' eighth and final season. According to Supervising
Director Dave Filoni, the story arc revolved around the possibility
that Ahsoka Tano would rejoin the Jedi Order. However, the
episodes were not completed prior to the series' cancellation in
2013.
The Siege of Mandalore first appeared in author E. K. Johnston's young
adult novel Ahsoka, which was released on October 11, 2016. For
her novel, Johnston drew upon plot points that Michnovetz had mapped
out with Filoni. Prior to then, Filoni had released a number of
details about the Siege of Mandalore in various forms, including
concept art as well as a panel presentation entitled "Ahsoka's
Untold Tales" at Celebration Europe in July 2016. Notably,
Johnston's Ahsoka explained how the Siege of Mandalore tied into the
events of the 2005 film Star Wars: Episode III Revenge of the Sith.
The novel also confirmed that Tano confronted Darth Maul on
Mandalore, an encounter that was first referred to in "Twilight of
the Apprentice", the second season finale of Star Wars Rebels.
At San Diego Comic-Con on July 19, 2018, Lucasfilm announced a
twelve-episode revival of The Clone Wars, which included the Siege of
Mandalore. When she wrote Ahsoka, Johnston was asked to not
include action scenes to not conflict with Lucasfilm's plans for the
season.
Source Wookieepedia
As explained above, Johnston took inspiration by the draft made for the Siege of Mandalore, and Disney asked not to include action sequences for the continuity sake, as they were to develop the new season wich will include the Siege itself.