T69959: The "ago" mechanism is untranslatable due to sentence split is still open and the callers are not moving. When can the "ago" system be used and why? Should it be replaced by getHumanTimestamp (although see T66430: getHumanTimestamp issues (tracking)) and when?

Document the difference and implement the best practice.

Choosing and documenting a standard is important to avoid undesirable inconsistency, especially when additional timestamps methods come into play (see T111596: Relative timestamp different between nojs and js mode ("1 day ago", "a day ago")).