This is not something which is controlled in the menus. It doesn't matter if auto assign is turned off, new settlers tend to be automatically assigned to plots. What I mean is, let's say you send a settler from one settlement to another and there is a free plot which is unoccupied, it may auto assign the settler to that plot. Same goes for new settlers which pop up in a settlement. I have auto assign turned off and I want to be able to assign my settlers each time without them being auto assigned because they are new settlers. The reason is, I tend to disable/markfordelete settlers I don't want and this can actually mess up the plots because if they are assigned to a plot then even when you disable/markfordelete them they are still assigned to the plot!