Rabbi I have a question which arose on this Purim past. I had the honor of shlishi when I completed the the brach I asked the chazan
to say “el molly rachamim” at which time he said that one cannot
say it on Purim.
The “El Maley Rachamim” prayer is not said on days that Tachanun is omitted, and this includes Purim.
See Iggros Moshe, Orach Chaim 2:74, concerning the question of why Shabbos special in this respect.