Tolkien created what could be considered one of the largest and most comprehensive worlds ever to fulfill fantasy. Of course, it's not so comprehensive that others can't take what was done and create their own creative interpretations of what happened.
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There is much more to The Lord of the Rings than the adventures of Bilbo and Frodo.
This is why every time a new piece of media based on Middle-earth comes out, people tend to question whether or not it's canon for the history of Middle-earth. The Lord of the Rings: The War of the Rohirrim, set in the history of The Lord of the Rings, is no exception to the question of whether or not it is canon.
Yes, Lord of the Rings: The War of the Rohirrim Is Canon
There should probably be a “technical” at the beginning of the “Lord of the Rings: The War of the Rohirrim is canon” sentiment because Tolkien is not alive to confirm or deny. However, based on the tidings he left behind in The Lord of the Rings, The Silmarillion, and The Unfinished Tales, The Lord of the Rings: War of the Rohirrim isn't completely removed from pre-existing canon lore.
The Rohirrim and the Riders of Rohan make appearances in The Lord of the Rings, but the best sketch of them comes from Appendix A and, briefly, Appendix B of The Return of the King. However, these outlines are not very substantial, which is actually to the benefit of creators as it gives them much more creative freedom to work with. As Jason DeMarco confirmed a few years ago, the creators tried to keep it as close to the source material as possible (though as little as possible).
As far as we can tell, the events that take place in the War of the Rohirrim do not conflict or contradict any other part of the LodR story.
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