Lock on targeting is included in the Vision Accessibility Preset and the Motor Accessibility Preset. It can also be activated manually from the Accessibility option in the Options Menu in the Alternate Controls section.
When set to ON, lock-on aim will target enemies who are onscreen. To lock on to offscreen enemies as well, select Lock-On Aim → Auto-Target.
The Aim Assist Slider does not affect Lock-On. Lock-On has a separate Lock-On strength slider directly below it to control the stickiness/distance of activation.
Lock-On for throwable weapons is Arc Throw Lock On. It must be enabled separately from Lock-On. It causes bricks/bottles to lock on slightly away from the target when in stealth to facilitate distractions.