The thing about the ancillary material is that it was supposed to fill in the gaps around the main story, delve into specific characters, tackle things that work better in prose than live-action or animated. It
shouldn't be changing up the model in which the story is presented by splitting it between media. As has been pointed out, there are enough people -- even serious fans -- who don't and/or can't follow all the ancillary stuff that comes out, or who don't even know it exists, which can only hurt audience numbers over time.
There's a lot, none from the films. Late Clone Wars introduced the "Force Tree" at the heart of the Jedi Temple on Coruscant -- apparently grown from a cutting from the one at the first Jedi Temple (seen in TLJ). Shattered Empire (comic miniseries) and Aftermath (novel trilogy) both start during the Battle of Endor, and follow their respective arcs forward into the weeks and months following. The former covers Operation Cinder and Luke recovering the remaining two cuttings of the Coruscant Tree from an Imperial research facility. One he takes with for when he founds his training center, the other he gives to Poe's mom (who volunteered for the mission) to plant where their family settled near the Yavin IV ruins. The latter, the other side of the Contingency, with the Emperor's protegé briefing Sloane on the plan and the preparations that had already been underway since Palpatine was Chancellor.
More got added in dribs and drabs in things like the campaign storyline of Battlefront 2 and a few other novels set between ROTJ and TFA, even if it wasn't the main focus.
Ah, that would be why it's called a contingency.

Plan for the worst-case scenario. One of the things about Operation Cinder is a corollary to the wiping out. The officers who received the orders from Palpatine's droid messengers also disseminated the orders that to say the Emperor was dead was treason for repeating Rebel propaganda. I wonder how much of that was an additional culling of the easily-swayed... as the Emperor actually
wasn't dead -- or, at least, had some method planned to return. We'll hopefully find out in a couple months.