Once considered career suicide, sabbatical are now not only accepted—but encouraged—by some employers.

= Once considered career suicide, sabbatical are now not only accepted, but encouraged, by some employers.

= Once considered career suicide, sabbatical are now not only accepted by some employers, but (also) encouraged by some employers.

为了避免重复by some employers, but encouraged 插入到前面accepted 与 by介词短语之间，表示二个被动语态的施事者相同，省略第二个by介词短语。