It used to be the case that, once the player had ascended, this feature was activated. It used to be necessary for the activation of Speedrun. Upon ascension, a player would have two savefiles - their normal file and their speedrun file. By default, the normal file would be unlocked and the speedrun file would be locked. A locked file could not be played. No more than one of the two files could be unlocked at a time. The Lock File feature allowed the player to lock their currently unlocked savefile so they could play the other.
This feature has been removed, as there is no longer any need to lock savefiles to use Speedrun.