Hi. The plunge is known and faster, however for Glitchless, where Ceaseless Skip is used, plunges as Fall Damage Cancel glitch are banned. 
The reason why the slope roll fall damage cancel is allowed is, with the same reasoning as toggling out of falls etc., that it can reasonably accidentally happen during a run at any point (think the drop down to the wheel in blighttown). Since we did not believe that plunge FDCs can happen accidentally these are banned.

See also https://wiki.speedsouls.com/ds1remaster:Any%25_Glitchless#Allowed_Mechanics

Dark Souls Remastered

Safer first drop for ceaseless skip.

catalystz

Beninho