Sunnah Tawatur And Ijma
Whenever new words are added or older words become obsolete, the same are documented and can be verified and traced back to their source. But language in general is not dependent on any book, neither is Sunnah. In case of any additions or changes, the same can be traced back to their source. Sometimes the source is undue influence of Ahadith (can be traced back), sometimes its just fiqh (can also be traced back to its source) and sometimes its only a biddah (whose time of initiation can also be easily known, for example, Eid Milad-un-Nabi as you mentioned somewhere else. Even the people celebrating it don’t disagree on it’s history). Sunnah being practiced among Muslims, just like analogy of language, doesn’t need to be looked at to be verified. If someone wants to look at the written history, they can refer to the books of fiqh (just like you refer to dictionary to look for obsolete and archaic words or sometimes for the etymology of any word). None of these activities challenge the authenticity of the original practice (i.e. Sunnah or language).