With the creation of the new theme-component category, rather than create another #broken-theme-component subcategory to go with it we have decided to retire the #plugin:broken-plugin and #theme:broken-theme subcategories in favour of tags that can span all three categories. It will also allow us to be more specific as to why a plugin/theme/component has been removed from the ‘current’ list.
For anyone Watching those categories for any reason, you will need to adjust your notification settings for the new broken, unmaintained, and deprecated tags instead. These tags will be muted by default to keep the topic lists uncluttered and up-to-date, but the topics will all still be accessible by filtering by the relevant tag in the dropdown.
Update: An end-of-life tag has now been added to the group. 
This will be used while a customisation still currently works, but will no longer be actively maintained. Unlike the other three, this one will remain unmuted. This will allow visibility for information purposes, and to give people a chance to be made aware that the customisation is indeed at the end of its current lifespan. After a reasonable amount of time, or until a breaking change occurs, it will then be removed from active view using one of the other tags.