I run KiCad v5.1.2-1 on W7.
On the Help file (pcbnew.html) there is, on the left column, an icon for:
“Enable/Disable automatic deletion of a track when it is redrawn” (BoardHelp.png)
(In the old days it was labelled: “Enable-Disable auto delete old track”.
As you see on “actualBoard.png” below, it is not shown on the board window.
But, based on Unwanted auto track deletion (solved) , it seems that the one mouse click of enable/disable is replaced by four; unless I missed something.
If this is the case, updating the paragraph “3.12 Left-hand side toolbar” on the next help file would minimize the confusion (for the old KiCad users in the least). And perhaps it would be good to add a note about how to do in order to get the same effect of the removed icon/function (menu: Route => Interactive Routing Settings).