I don’t know how to get the previous brush.
It seems impossible to share.
What I can confirm is that the brush management method has changed since blender 4.3, and the newly created brush is automatically saved in the library.
I think we should set up a new brush. 
Add…
How to fix the problem.
- Change the view in the outliner to blendr file.
- Open the brush list and register the required brush as an asset.
This will make it available in blender 4.3.
Brushes registered as assets are registered for immediate use.
Also, there is no need to reload them in new projects.
If you back up and move files saved as assets, you can use them the same way on other PCs.