That's just TOS books. Not Star Trek novels in general.
Sorry, I meant to re-write that. I first read the question as being about the series, and forgot to clarify why I was only talking about TOS.
But it's still been fifteen years (and thirty-three novels). In retrospect, female authorship seems to have first declined sharply after 2001*, then ended after the Great Recession.
It's baffling for a series that's famous for its great female authors (Duane, Crispin, Carey, Fontana, McIntyre, Kagan, Lorrah, Dillard,
garamet, Ecklar, etc.).
*Until 2002, most TOS books were female-authored, but only five of twenty-five (

) in the next eight years.
As noted, folks like Una McCormack, Kirsten Beyer, Cassandra Clarke, etc. have been writing books for TNG, DS9, DISCO, PICARD, SNW.
And that's not even counting various short stories in Star Trek Explorer magazine and the hardcover collections of same.
Among female authors, only
Una McCormack has published a novel for the franchise in the last four years. In the last fifteen years, only three of twenty-three novelists have been female.
As few as male authors increasingly are in publishing broadly (which should concern the industry), and as popular as Star Trek has always been with female writers,
something has obviously gone wrong in either the pipeline or the process.
The last fifteen years have seen more men named David than women (I'm sorry to see that Dave Galanter died; I'd heard about
LoneMagpie):
Female authors
- Cassandra Rose Clarke
- Kirsten Beyer
- Una McCormack
Male authors
- Alan Dean Foster
- Alex White
- Christopher L. Bennett
- Dave Galanter
- David A. McIntee
- David Mack
- David R. George III
- Dayton Ward
- Greg Cox
- Kevin Dilmore
- James Swallow
- Jeff Mariotte
- Jeffrey Lang
- John Jackson Miller
- Michael A. Martin
- Michael Schuster
- Steve Mollmann
- Peter David
- Tony Daniel
- William Leisner
In addition to the three cited by GC, I think Paula M. Block would also have something to say about
@Cicero's assertion.
Do you mean her short fiction from eight years ago?