You know it is protocol to address a former elected official by his highest achieved title But is it necessary when referring to them in writing?

Answer 1

When and where it is 'protocol' to address a former elected official by his/her highest-achieved title, it will always be appropriate (as an observance of protocol and, even more, as a sign of respect) to address that official in written correspondence by that same title. Whether it is necessary depends on the particular circumstances of the context of the correspondence; for example, where the official and the correspondent are on familiar terms, there is no need to make use of the official's highest-achieved title.