Some have noticed that of the final 3 Little House on the Prairie movies, the three two-hour movies that comprised of the final 10th season, that Katherine MacGregor, who portrayed Harriet Oleson on the show, did not appear in any of these final episodes.
Richard Bull, who portrayed Harriet’s husband Nels Oleson, did appear in these 3 movie episodes and is one of only a handful of characters to appear in every season of the show.
For the final episode, where Walnut Grove is blown up, there are many other significant characters who did not make an appearance, including Nellie Oleson Mary Ingalls and her husband, Carrie, Grace, Albert, James and Cassandra Cooper. However, most appeared in one or both of the other two television movies that aired as the 10th season. But because Nels appeared, it was more obvious that Harriet was missing.
As for specifically why she was missing from all three episodes, it was reported that Katherine MacGregor was on a pilgrimage in India at the time, so she wasn’t available for filming.