Version 5 introduced the MyPaint brush engine but a brush engine isn’t something you download, it’s built in.
There are some Mypaint brush presets that you will see in the Brush Presets docker if you type “mypaint” in its search box.
After that, it’s up to users to create MyPaint brush engine brushes and make them available for general use. e.g:
Fizzyflower's ULTIMATE MyPaint Brush Collection
That is a ‘megapack’ of many different mypaint brush bundles and quite overwhelming.