The list of areas that have become flexible has grown more and more over time, such that even a
Narcomeoba would be cut from time to time. I don't believe that all of this "flexing" is beneficial, though - in fact, I find that most of it is probably harmful (the
Narcomeoba example being one of the more obvious ones). As an example, I find cutting a
Breakthrough or two from the main deck to be questionable, as my experience has taught me that I would want a
Breakthrough in my hand as often as possible. The same goes for
Cabal Therapy, especially when considering the possibility of going against tricky match-ups. Of course, it has been proven time and again that running less than a full set of these cards can still produce wins, and that's where things become much more "flexible", at least in terms of perception.
Virtually every card in the deck has been "flexed" at one time or another, except maybe for the following: