There is no custom that restricts Ashkenazim to eating hard Matza. Although Ashkenazim have eaten hard matzoh for many years, the practice has the same significance as the practice of making parochet of blue material rather than of red or maroon material.
The above letter from Rabbi Hershal Schachter was sent out attached to the following email from Rabbi Meir Gershon Rabi, who runs a kosher supervision agency (hashgacha) in Australia:
Shalom to you, my Friends
I am pleased to send this note from HaRav Schachter, the Rosh HaYeshiva of YU and senior Halachic advisor to OU Kosher.
A summary of HaRav Schachter's note.
There is no custom that restricts Ashkenasim from eating hard Matza.
Although Ashkenasim have eaten hard Matza for many years, such a practice has the same significance as the practice of making the Parochet of blue material rather than of red or maroon material.
Besides, there is evidence from the Mishna Berura that "Matza made as soft as a sponge" can be used for the Mitzvah of eating Matza.
Furthermore, the Rema who mentions that Matza should be made as Rekikin, means to say that Matza should be made thin, even less than an Etzba (about 12mm), not one Tefach thick (about 8cm) as permitted by the Mechaber. He does not mean that Matza should be made as thin nor as hard, as wafers.
Rabbi Meir G. Rabi