Finally season eleven is this the season Danny and Kathy go to Europe??
No, they went to Europe in Season 10.*
*I answered this at the exact time AlanP reasked it in a slightly different way!
About the Hagen Episodes:
It seems to me that somewhere along the line, maybe starting in Season 8 (1960-61), after Terry disappeared, never to be seen or mentioned again, the show behaved as if Danny had always been married to Kathy and she was the mother of Rusty and Danny was the father of Linda. That might have been one motivation to pretend the Hagen episodes never existed.
However, if that's why the first four seasons (the three Hagens and the "widower" season) were removed from syndication, then logically, seasons 5, 6 and 7 should have been, too, when Rusty called his stepmother by her first name.
I sincerely wish this was my biggest problem in life, but it's interesting to speculate.