How accurate is the saying that there are abandoned Sunnahs which only those upon whom Allah has bestowed His mercy practice?

If the intention behind these words is that there is a forgotten Sunnah that only a few people practice, then there is no harm in it. This applies to many Sunnahs that people neglect or do not know are Sunnah, such as offering sacrificial animals (bringing Hady) to the Ancient House (Kaaba).