To Yanky55,
Thank you for encouraging me to look it up. Indeed, the Aruch Hashulchan mentions it twice (OH 66:16 and OH 111:2)and clearly states that the minhag is not to say amen after ga’al yisrael, against the Rama, but in accordance with the Mechaber and the Zohar. It is easy to see how this minhag encouraged shlichei tzibbur to lower their voices so as not to encourage answering amen. As I mentioned before, when I was a child, with the many shuls and dozens of shlichei tzibbur that I heard, the shatz always lowered his voice or was silent for the last two words of the bracha. I started to say this bracha out loud in high school (and have continued since), but my rebbe and rosh yeshiva, a brilliant lamdan and known iluy in the Telz yeshiva in Cleveland in the fifties, really let me have it when I did it as a shatz in school. He told me he knows what the Rama says and that is NOT the minhag that is practiced now. He saw it as gaivah on my part to go against the prevailing minhag.
In any case, the sefer Tefillah Kehalacha ( this sefer has haskamos from Rav S.Z. Auerbach zt”l, Rav Elyashiv shlita, the Minchas Yitzchok zt”l, Rav Scheinberg shlita, Rav Halberstam, and Rav B.Z. Abba Shaul zt”l, certainly an All-Star lineup if I’ve ever seen one) mentions that Rav Henkin zt”l railed against chazanim who said the last two words silently. Obviously the minhag was prevalent if he felt the need to make a big deal out of it. In addition, Tefillah Kehalacha hints that the Chasan Sofer (Rav Shmuel Ehrenberg, grandson of the Chasam Sofer and great grandson of Rabbi Akiva Eiger) mentions this minhag. Although the Chasan Sofer is currently unavailable to me,I will try to check it out.
Since the poskim generally were not in the habit of writing detailed descriptions of what exactly happened in their davening, it is not surprising that the specific information on this minhag is lacking. The Aruch Hashulchan does it occasionally, and the Mishna Brurah almost never.
Hope this helps clarify things a bit.