I agree with Frederik. This request is actually for Vector Brushes and Vector Layers / Vector Modes. I would not expect Raster Brushes or Raster Layers to be below one pixel.
With Vector Point Sizes (not pixels), I would expect brushes to go below 1.0, just like other vector design programs can do such as Clip Studio, Adobe Illustrator, and Corel Draw. In fact it might be good to have the option to change from Pixel mode to Vector/Point mode, so you can do this separately.
What is interesting is that you can currently select your line artwork and choose to scale it below 1.0 point sizes under the Stroke tab (see attached image); so this functionality is somewhat already there in AD. It just sucks to have to draw artwork in a larger size format and then manually change all the brush strokes to be below 1.0 stroke point size.
With near infinite zoom in AD (which is super awesome), this 'below 1.0 scale' functionality is really necessary to get your detail work in without having to scale everything up by 1000x.