Documentation
The record of a language — texts, recordings, grammars, dictionaries — and how much of it exists.
UNESCO treats documentation as a measure in its own right — “a guide for assessing the urgency for documenting a language” — and names what counts: “Of central importance are written texts, including transcribed, translated, and annotated audiovisual recordings of natural speech.”
Note transcribed, translated, and annotated. A recording nobody can read is a weaker record than it appears; the work of making it usable is most of the work.
Documentation is what decides whether a language can be awakened later. The same document is stark about the alternative: “Without adequate documentation, a language that is extinct can never be revived.” It is also the factor that a community and an outside linguist are most likely to weigh differently, since it measures the archive rather than the life of the language.