In systems with non-voting nodes, confirmed w:majority writes might in some circumstances still roll back. This is because rollback is precluded by ensuring that a majority of voting nodes are aware of the write, but w:majority in a system with a high ratio of non-voting to voting nodes might not ensure this.
Conversely, on clusters with a high non-voting to voting ratio, w:majority is really too strong a requirement for committedness. Only the majority of the voting nodes need to respond, to be safe from rollback.
The work of this ticket is to change the meaning of "w:majority" to indicate acceptance of a write from the majority of voting nodes. No built-in write concern will provide the old functionality.