Texture system.png is inconsistent with the rest of the GUI.

This too is a change request. They're different styles of toasts as they communicate different things.
Straight-corner ones communicate more important matters (backsups, how to play, warnings), while rounded-corner ones are just a "good job" which you can see elsewhere too (advancements menu, chat announcement, recipe book).
That they're not the same is a design choice mojang made. Half of the toast types are square while the other half are rounded. There's no true inconsistency here. This is just asking for a change.