It was an idea of Sam’s (and maybe Dipak), from close to 20y ago. It was something I think they thought would generalise across medicine, but over time the only examples that have ever emerged always seem to have been patient position during BP measurement and exertion during heart rate measurement.
For good or bad, we have engineered the capability to have ‘assumed values’ in any primitive data type. This could potentially be used as is to implement some sort of default values, but since default values need to be definable for larger granularity objects (potentially even up to whole Observations) it might be better to deprecate it.