https://www.metv.com/stories/heres-why-robert-reed-wasnt-in-the-final-episode-of-the-brady-bunch

Reed had decided enough was enough.

And by that time Sherwood Schwartz had decided the same thing.

He also boycotted and refused to appear in Goodbye Alice, Hello because he was so against the script thinking the kids would never treat their housekeeper that bad. He also refused to appear in the final scene in Welcome Aboard with the pie fight when they visit the movie studio with Oliver.

He ruled.

At the end of Season 5 of The Brady Bunch, Robert Reed wrote an infamous memo (https://www.ericdsnider.com/misc/batman-in-the-operating-room-why-some-comedy-isnt-funny/) to Sherwood Schwartz in which he complained about some elements of recent scripts for the show. Specifically, he was unhappy with what he thought was the insertion of surreal, zany, unrealistic, and slapsticky plot elements. He felt they conflicted with the established tone of the series, which heretofore had been light comedy-drama that was basically grounded in reality.