**update** since the question was clarified later, some parts of it no longer fit.

However that *probably* isn't Endor because the Ewok film showed Endor as light gray as seen below. Not red.


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1. According to illustrations in *The making of Star Wars: Return of the Jedi*, it was indeed the forest moon of Endor. Or simply "Endor", as the name of the moon kept changing during production.

   Here's the book's illustration titled "Chris Evans at work on a matte painting of the forest moon of Endor":

   [![enter image description here][1]][1]

   And here is the shot with the planet looking nearly identical to one in your question, from a series titled "Final frames of the first five matte paintings of the film, by Pangrazio (#1=the curve of the forest moon)": 

   [![enter image description here][2]][2]

   * Additional evidence that it is not Endor the planet: the planet is a **gas giant**. This doesn't look like a gas giant.

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2. I'm still looking but I don't think Endor (the gas giant planet itself) appears in the film

   Please note that the iconic image gracing both [Wookieepedia page][3] as well as [this SE question][4] is as far as I know sourced from Ewoks film (Caravan of Courage?) and not ROTJ!

   [![enter image description here][5]][5]
 


  [1]: https://i.sstatic.net/vLHXJ.png
  [2]: https://i.sstatic.net/uKCjy.png
  [3]: https://starwars.fandom.com/wiki/Endor_(planet)
  [4]: https://scifi.stackexchange.com/questions/34376/is-there-an-astronomical-name-given-to-the-ewoks-home
  [5]: https://i.sstatic.net/J9zWv.jpg