When in history did we start saying the "harachamans" at the end of benching (Birkat HaMazon)? Are there communities that don't say them? Are they obligatory?

The "HaRachamans" are not found in the ancient bentching found in the Cairo genizah, but are already found in the siddurs of R. Amram Gaon in Bavel, about 1150 years ago, and R. Saadya Gaon. Accordingly, as far as I know, they are said, with minor changes by all of the communities, and as all customs, obviously should be said, unless one is really in a hurry. They are also mentioned in the Tur (Or.Ch. 189) from about 650 years ago as a custom which is commonly known, and he suggests that it may stem from the bracha of the guest for his host which we recite after the 4 basic brachot, which is already found in the Talmud (Brachot 46a). Rav Ari Shvat (Chwat)