Filtering by “no tags” behaves as if a tag literally called “none” is used.
The strings for js.tagging.filters.untagged_without_category and js.tagging.filters.untagged_with_category do not seem to be applied.
A js.topics.bottom.tag variant would probably be useful, too.
Attempting to create a topic when the “no tags” filter is set results in an error.