An expanded enhancement request has been filed here: https://bugs.freedesktop.org/show_bug.cgi?id=87628. It includes the following question:
Currently, the Word Count tool in Writer measures only the number of words and characters. Is there an easy way to add a “words-per-sentence count”?
In other words, I’m suggesting a new feature—let’s call it Wopse—that would measure the average sentence length in a given text. Writers could use this tool to write more effectively by keeping their sentences shorter (e.g., at 20 words or less).
An online service providing words-per-sentence count is available at https://readability-score.com/. The website provides other measurements as well and is based on the open-source Text Statistics project (GitHub - DaveChild/Text-Statistics: Generate information about text including syllable counts and Flesch-Kincaid, Gunning-Fog, Coleman-Liau, SMOG and Automated Readability scores.).