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.
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.