After using Glorfindel's excellent answer for several months, a few improvements have been added.
After some use, it became evident that
WikiPostId might be non-null, but the body of the Excerpt or Wiki would still be blank. This could probably be caused by a number of things. (In my case, a few of my tag edits were rejected, but the tag in question was still removed from the result set even after the weekly SEDE data update.)
The improved query takes into account null IDs, an empty body, and removes tag synonyms.
For reference, here is the complete query: Tags Without Excerpt or Wiki
SELECT a.TagName, a.CountPosts AS 'Number of Posts', a.Excerpt, a.Wiki
SELECT c.TagName, c.CountPosts,
CASE WHEN (c.ExcerptPostId IS NULL OR LEN(LTRIM(RTRIM(e.Body))) = 0) THEN 'N' ELSE 'Y' END AS 'HasExcerpt',
CASE WHEN (c.WikiPostId IS NULL OR LEN(LTRIM(RTRIM(w.Body))) = 0) THEN 'N' ELSE 'Y' END AS 'HasWiki',
e.Body AS 'Excerpt', w.Body AS 'Wiki'
SELECT t.TagName, t.ExcerptPostId, t.WikiPostId,
COUNT(*) AS CountPosts
FROM Tags t
LEFT OUTER JOIN TagSynonyms s ON (t.TagName = s.SourceTagName)
INNER JOIN PostTags p ON (p.TagId = t.Id)
WHERE s.ApprovalDate IS NULL
GROUP BY t.TagName, t.ExcerptPostId, t.WikiPostId
LEFT OUTER JOIN Posts e ON (c.ExcerptPostId = e.Id)
LEFT OUTER JOIN Posts w ON (c.WikiPostId = w.Id)
WHERE c.ExcerptPostId IS NULL OR c.WikiPostId IS NULL
OR LEN(LTRIM(RTRIM(e.Body))) = 0
OR LEN(LTRIM(RTRIM(w.Body))) = 0
ORDER BY a.HasExcerpt ASC, a.HasWiki DESC,
a.CountPosts DESC, a.TagName ASC