The nomenclature, as commonly used isn't precise enough.

18650 means 18x65mm, but "protected 18650" usually means that cell with electronics stuck on the end, which ends up at 68-70mm long. Nobody calls the result "18680" even though that describes it more precisely.

There isn't a common cell slightly shorter than 18650 to produce protected cells that fit in every holder designed for 18650. The AA-size (but not AA-voltage!) 14500 often does come in protected versions that are exactly 50mm long, based on 14430 cells.