It seems to have been merged into 'Black Hair' but these are two different things. First of all black hair doesn't specify gender and secondly dark hair applies to more colours than black.

Missed 'the' before 'tag' in the title, but there doesn't seem to be a way to edit it, sorry you have to see such an annoying error.

https://www.mangaupdates.com/series.html?category=Dark-Haired+Female+Lead

This one?

I think being able to separate between genders for hair color is important.

There are a lot of color Shoujo/Josei Manhwa that have come out. They tend to have very similar plot lines and identical genre tags, so when trying to find a specific one being able to search for specific coloring is important (since it is something people often remember in a color comic)

I know that the black-haired male lead with red eyes is one of the combinations that gets a lot of attention in comment sections.

Edit:
Another reason is that sometimes people go looking for a story after seeing a screenshot (examples: in a comment section, on social media, etc).

I started adding the hair and eye color for color Manga/Manhwa/Manhua after seeing so many people have the character physical descriptions on the 'looking for' forum on this site. (If all characters have dark hair then I don't add the Category since it is not notable)

Before removal, the category had close to a hundred titles. Now it seems people have been adding it back slowly.