Hi Atlassian team,
As a software developer of Communardo, vendor of the Metadata for Confluence app, I would totally agree with the previous feedback already given here. There would be significant blockers for our app in the proposed changes.
Deprecation of the atl.page.metadata.banner
Our users would be interacting with a panel in the form a button inserted into atl.page.metadata.banner extension point, that triggers the opening of a dialog used for assigning/editing metadata.
Our major concern is usability. Not only are the customers accustomed to the existing way of interacting with the app but changing it into a Byline item would mean reducing visibility, not being easily noticeable and therefore preventing our app from effectively communicating with the users.
Currently, the panel itself displays dynamic information. Because you recommend a limit of 20 characters for the Byline item, we are forced to change again the feature of our app and this way we would hide more information from the users.
Deprecation of system.editor.precursor.buttons
The functionality explained below is similarly implemented from the page editor side, adding to the system.editor.precursor.buttons extension point. You mentioned that the updated Byline designs should be able to cater for the deprecated locations. Does this mean that the Byline location would be present in the Confluence page editor? If yes, how does that work, the same as for the page rendering view? Maybe sharing a screenshot for the page editor part, would help.
Deprecation of the atl.editor.savebar
We would need an alternative for it because we are using the location as a substitute for a background running script. In the current solution, we could execute the script in the context of the content in which it is running, we have access to the currently logged user, and we are able to show a flag notifying the users.
Considering atl.general location as a substitute, you have informed us that the atl.general is going to be included in the editor of a page as well. But, as others have already mentioned, users would get confused a lot if they see a loading spinner at the top of the page and then not render anything.
Sidebar overflow menu
We are using the current system.content.action/modify location so that the user can interact with the app easily. As for the change on consolidating the existing extension points into a menu item ‘Apps’, it would contribute to further hiding app functionality, diminishing the user experience overall. As a user, I would get confused not seeing the buttons anymore on first level of the menu. Also, it can happen that a user could not be able to identify which button comes from which app and get furthermore confused.