On Itch IO, there are preset tags already which automatically treat singular and plural words as being identical tags. For example, searching for "cat" or "cats" as a tag yields the same results. But for unofficial tags that don't quite convey fully a certain sub-genre, oftentimes hundreds of games become divided over trivial nomenclature such as being singular/plural or whether a word should be hyphenated or compound instead.
As a quick example:
Searching for "monster-girl" gives 115 results.
Searching for "monster-girls" gives 76 results.
Searching for"monstergirl" gives 58 results.
Searching for"monstergirls" gives 25 results.
For the most part all of these games just use one tag, so they are not easily cross-recommended by the algorithm or show up as search results when people try to browse games of the same genre. I did an experiment and messaged a few developers by recommending them to switch which tags they use to the most popular version and in some cases they claimed it doubled or even tripled their metrics (huge when the topic in particular happens to be a niche, but devoted fanbase).
The website should probably automatically lump together words it can identify as plurals of each other. If possible it should lump together hyphenated and non-hyphenated words automatically if they would be identical without the hyphen. Most immediately though, I'd recommend that those 4 tags in the example above be aliased together at least and possibly be made an official tag (big sales happening right now there). There are other direct examples I've seen over the years, but they escape my mind at the moment, and in terms of raw numbers this was a particularly egregious example since it currently covers multiple hundreds of titles.
-Best Regards