Just seen the publish dropdown has been replaced by a sidebar panel #4119, as first step towards the updated publish flow #3496 (comment)

This is a very good example of why accessibility specialists recommend to design with accessibility in mind since the beginning. I see this new feature as an UI improvement compared to the previous dropdown but I also see many accessibility concerns haven't been addressed at all, not even the most basic ones. At this point of the project, I'd expect to see new features merged with some basic accessibility already built in.

A few things noticed so far:

Placement:
The publish panel is placed outside of any of the 3 landmark regions, and also can't be navigated to with the Ctrl+backtick shortcut. Since the introduction of landmarks to indicate 3 main editor regions (toolbar, content, sidebar), any UI component must be placed within one of these regions. Landmark regions can be reviewed and adjusted, of course.

Keyboard navigation:
Activating Publish/Update in the toolbar makes the Publish panel slide in but there are several tab stops between the toggle button and the panel. As mentioned several times on other issues, see for example #469 (April 20th!), controls that expand a panel should be placed immediately before the panel itself, or focus should be handled programmatically. The first option is always preferable. Now, when the publish panel opens, the underlying default sidebar controls are still focusable, though visibly hidden. This produces various tab stops to hidden controls and must be avoided.

Focus loss:
Closing the panel by publishing/updating/switching to draft, makes the panel disappear and there's a complete focus loss: press Tab again and keyboard navigation starts again from scratch from the document root.

Also, disabling focused buttons causes a focus loss UI controls should never get disabled while they're focused. In this kind of scenario, either focus should be moved programmatically to a logical, predictable, expected, place, or the control shouldn't get disabled in the first place. Worth reminding in at least one other place, we're keeping a button focusable and just make it noop.