That's not a brush. It's a complex non-repeating pattern along the wanted outline. It can be a real painting which is photographed and colored. I can easily believe just this to be done.
If it's created in PC, it can have been painted at first with solid line which is made ragged. The ragging needs some idea of the wanted result. I show only a way to get easily something random, but rich enough. This is a raster image in Photoshop.
Have a solid thick black line on white (a single layer image, no separate background). In practice this shouldn't be straight, but the wanted shape curve.
Take some present scatter brush and paint black and white over and around the black line:
Using the smudge tool with some multi-dot brush push and pull the original straight edges invisible. You can also paint some spatters. They seem to be essential in your example. Finally add very slight gaussian blur:
Goto Image > Adjust > Treshold and adjust the treshold of black and white for good effect:
You may not want as many or as big holes. Copy pieces from elsewhere to fill the exessive holes or make them smaller. Remove white with magic wand and pressing DEL. Then you can give to it any color with Image > Adjustment > Hue/Saturation > Colorize
My pattern unfortunately is only a straight line which has artistic value = zero. But it can be easily used in illustrator, Inkscape, Affinity designer etc... as an image pattern brush or a shape which is bended along a curve. As a brush it can be used for painting:
Just in this case the pattern was traced in Inkscape (a 3 second job) and copied to the clipboard. Then a shape was drawn with the freeform pen. The pen was set into mode "bend from clipboard"