Star Wars: The Clone Wars season 7 is foreshadowing the deaths of some Jedi characters who are killed by Order 66 in Star Wars: Episode III - Revenge of the Sith. Now entering its final stretch of episodes, The Clone Wars season 7 is reaching the same point in the timeline as Revenge of the Sith. The Siege of Mandalore arc that will wrap up season 7 happens concurrently to other battles in the final days of the Clone Wars, and when the siege ends, Order 66 will have been given and and the Jedi will be no more.
The Clone Wars is by no means the first Star Wars story where audiences have a good idea of where the story is going before it's finished. The whole Star Wars prequel trilogy, for instance, leads to a predetermined end necessary to set up the original trilogy. And more recently, Rogue One: A Star Wars Story follows a team of Rebels as they steal the Death Star plans it was already established they would successfully steal. Still, none of this foreknowledge really hurt the enjoyment of either the prequels or Rogue One, and so it is with The Clone Wars. In fact, season 7's most recent episode makes a point of directly referencing the impending deaths of two Jedi, using viewers' knowledge of what's to come to help relay how close the cartoon now is to the events of the movie.
In Star Wars: The Clone Wars season 7, episode 9, "Old Friends Not Forgotten", the episode's opening reel quickly sets up the current state of the galaxy, highlighting skirmishes like the Outer Rim Sieges. During this intro, it's also explained that the Jedi council is dispatching their generals to bolster the clones fighting far from the core worlds. The two Jedi who are shown in this sequence are Plo Koon and Aayla Secura, and both are seen arriving at the planets where they will later be killed by their clone troopers in Revenge of The Sith; Plo Koon flying in a fighter above Cato Neimoidia and Secura entering the forests of Felucia.
The events of Revenge of the Sith have hung over the entirety of The Clone Wars. This is especially true in regard to Anakin, with so much of his journey in Clone Wars further selling the tragedy of his fall to the dark side. Order 66, however, has also been heavily foreshadowed throughout The Clone Wars, and it along with Anakin's killing of the younglings is what signals the time of the Jedi has ended. Those moments in the Star Wars saga are now quickly approaching, as Plo Koon and Aayla Secura leaving for their final missions only emphasizes.
The quick cutaways to Plo Koon and Aayla Secura are even more effective in the context of Clone Wars. Unlike in the prequel movies, where so much screen time needs to be dedicated to telling the main story of the Republic's collapse and the rise of the Empire, The Clone Wars has been able to spend whole episodes and arcs focusing on lesser known but no less important characters. Plo Koon, for example, is the Jedi who discovered Ahsoka Tano, and Aayla is shown working alongside Anakin and Yoda on more than one instance. When these Jedi are killed in Revenge of the Sith, however, they're little more than background characters who die as a plot point in a larger story. But on Star Wars: The Clone Wars, when characters like Plo Koon and Aayla Secura are killed, it will matter to the characters who survive them, like Ahsoka, and it will matter to audiences who've gotten the chance to know them across seven seasons.