This hook, documented as Use this hook to add an edit toolbar above the textarea in the edit form., basically serves no purpose anymore.
It allows extensions to add a custom edit toolbar, which will be wrapped in a div with the id toolbar, one of the legacy styling to be removed (T89981). This legacy styling also limits the bar to 22px, causing a FOUC when not exactly 22px high.
In its current usage it is only ever used to disable itself, which is pointless - when no hook modifies the toolbar, it is not shown (T30856).
Extensions that want to change the edit area use different hooks, given that none of the popular editor extensions (VisualEditor, CodeEditor, WikiEditor etc) are listed.