Perhaps a rarepair category, where ships with little to no fanworks are categorized. Like Fitzaby said, it's impossible to know the amount of shippers there are for each ship. Not only that, but some ships may be more popular on certain platforms than others, which makes it even harder to determine a certain number/percentage.
A ship can be given the rarepair category if it has less than three different authors on AO3, or it doesn't have a tag on Tumblr. This of course doesn't apply for ships from new fandoms, which it may take time for fanworks to flow in, so the pairing has to already been introduced to the series/fandom in canon for at least 4-6 (someone decide on the number) months. After which if it still has very little fanworks then it can be considered a rarepair.
This was just a suggestion, personally I feel like it's fine as it is, as a ship can always be mentioned as a rarepair in the Fanon section of a ship page. But I guess for those specifically looking for rarepairs in a fandom (which I'm assuming is what the OP is asking for), it'll be easier for them if there was a category.