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.