The Final Richard
The show never alluded to Dick York’s departure, and instead simply hired a new actor as a replacement. York was missing from several episodes in season three and eventually stepped away from the show due to health complications. While originally unplanned, there were multiple episodes with no Darrin Stephens, and Elizabeth Montgomery stepped up to fill in for the missing character, sometimes by appearing as her identical cousin, Serena.
Plot-wise, Darrin’s character was written off as away on business, but in reality, York was struggling with chronic back pain and unable to film all episodes slated for the series. Apparently his injury resulted from a stunt performed in a movie. Dick Sargent was hired to step in as Samantha’s mortal husband, and took on the leading role for the remaining five seasons.