Grease Pencil 3.0 was going to be included in the LTS release which preceded 4.3, but various complications prevented it from being ready for the main branch. Needless to say, it is finally HERE. In spite of the changes, there's a lot that is still familiar, and you won't need to re-learn a great deal in order to use it.
Brushes
One big change is how brushes are stored, accessed and shared. They are now part of the asset system.
Because they are saved as assets, they can now be accessed in new .blend files.
Tools, Menus, where to find things
The Draw toolbox has been re-organized, grouping together tools into handy pop-outs.