If you’ve started using Photoshop CC 2018, you may have noticed a strange purple line following your brush around as you’re using it now. That line is called a “brush leash,” and here’s a helpful 6-minute video by retoucher Pratik Naik about what that purple line is and how you can use and customize it.
Basically, the purple brush leash is meant to serve as a guide for the Smoothing feature of the Brush Tool. Turning Smoothing down to 0% removes the purple line and restores the brush tool to the original version.
You can also turn off the brush leash in the Cursors panel of your Preferences.
But if you like the effects of smoothing out your strokes, you can customize the brush leash color in the same panel to whatever color you’d like.
In summary, the new brush leash line is meant to serve as a visual guide for your brush smoothing and is on by default, but you’re free to remove or customize it by adjusting smoothing and in your preferences.