The documentation is missing for Panel & Tabs starting in version 1.1.0.
Since updating, I have noticed it is throwing an error when inserting a width to a panel, although width is in the JSON Prototype (at least in the doc for 1.0.4), I couldn’t find any mention to it in the release changelogs.
The physical file location was changed to bokeh.models.layouts in the 1.1 release, though transitive imports were added to bokeh.models.panels to maintain compat. It’s omission the the docs was an oversight, though note in the upcoming 2.0 release we intend to collapse all the bokeh.models docs and no longer advertise the physical file structure any longer. (i.e guidance is that users should import everything from just bokeh.models and not rely on submodules)
The 1.1. release was a huge overhaul of the layout system and it took things from “mostly not working in many situations” to “mostly working”. It looks like Panel is no longer a LayoutDOM which it why it does not use a width any longer. It should have never had a width to begin with, because a Panel should always fill the tab that it inhabits, in all cases. However, we should have added a no-op property getter/setter for width that explained that the value was not needed and ignored, rather than tripping an exception. Also an oversight. My apologies, but we could definitely use help and resources testing, especially when there are larger chunks of work.