But I agree with other comments in this thread that the style imbalance doesn't really have anything to do with a feat tax. The current rules don't even include primary combat feats, so it's not really practical to evaluate whether as-of-yet unpublished feats are so good as to be effectively mandatory.
I think that we're using different jargon here. My understanding of the orthodox usage of "feat tax" is: A required feat that designers introduce in order to make up for a design oversight. Hence you must take this feat to equilibrate your character build with mean performance expectations. Other builds are not "taxed" this feat and can thus diversify or focus their characters further rather than spending a feat to achieve mean performance expectations. Or, PCs generally have to take the feat in order to keep up with challenge target numbers. Hence it is a tax.
Case in point in 4e:
Problem #1: At approximately levels 5, 15, and 25, the attack bonuses of player characters gradually begin to be unable to keep up with monster defenses, thereby causing a deterioration of player character hit probabilities as the levels rise.
Solution: Introduce the Implement Expertise and Weapon Expertise heroic feats in the Player's Handbook 2, and later, the Focused Expertise feat in Dragon Magazine #375.
Problem with the Solution: Pointlessly fills up a feat slot that could have been spent on something other than patching an issue caused by an oversight of the game designers. [As an aside, characters are now strongly discouraged from ever using an implement or weapon not covered by the respective tax feat, and characters who use two different types of weapons or implements (such as a paladin with both a magic weapon and a holy symbol) are unfairly forced to take two iterations of these tax feats.]
Problem #2: At approximately levels 16 and 26, the Fortitude, Reflex, and Will defenses of player characters gradually begin to be unable to keep up with monster attack bonuses, thereby causing an increase in monster hit probabilities against these three defenses as the levels rise. This is due to masterwork armor granting an overall +1 bonus to AC at level 16 and then another +1 AC at level 26, but failing to commensurately increase Fortitude, Reflex, and Will.
Solution: Introduce the Paragon Defenses paragon feat and the Robust Defenses epic feat in the Player's Handbook 2.
Problem with the Solution: Pointlessly fills up a feat slot that could have been spent on something other than patching an issue caused by an oversight of the game designers.