That is no plugin, the MyPaint-Brush Engine is an implemented part of Krita, you have it already. A brush engine would not work as a plugin, a plugin would not be fast enough for the requirements brushes have. That has to be coded in C++, or similar powerful programming languages, python, for instance, won’t do.
Enlarge your brush editor to the left, fold out the view of brush presets, in case you folded it in, and then open the drop-down menu “Engine” and select the option “MyPaint” to see at least those few Krita ships with. And if you are interested in more of them, you may take a look at Fizzyflower’s ULTIMATE MyPaint Brush Collection or maybe at URAVERAGEBOI’s Mypaint brush-, or try Using a mypaint tip gives krita brushes a mypaint feel, or the Paint and Blend - An Attempt at recreation of “Paint and Blend” MyPaint Brush and all the other bundles not explicitly naming that they also contain brushes using it.
Michelist