One of the longest-standing rumors regarding Yu Yu Hakusho is that Kurama and Hiei were originally planned or supposed to be an example of a romantic couple by Yoshihiro Togashi himself. Is it true?
Well, the short answer is: No, it's not true. There is no evidence, interview, or footnote by Togashi stating anything of the sort.
The long answer is: Although the rumor is false, it isn't very surprising. In fact, it makes a lot of sense as a conclusion when you analyze the body of fan works surrounding Yu Yu Hakusho, and also Togashi's personal interests and inspirations.
Mori over at yyh4ever has a wonderful and thorough post
explaining precisely any and all descriptions of Kurama and Hiei's relationship and creation for the manga, which is relatively unexciting. That is to say, there isn't much explained by Togashi that isn't apparent in the series itself.
So where did this rumor start? I think the highest probability comes from the artwork following the anime release by Mari Kitayama. She was the lead character designer of Yu Yu Hakusho's 90s adaptation, and drew a large amount of works people have come to recognize from the anime.
A cropped official work drawn by Kitayama.
She also goes under the pseudonym "Maririn", and is a doujin artist. Of note, she has drawn multiple works involving Kurama and Hiei as a romantic couple, and published numerous unofficial drawings of the series and it's main cast.
From 'Rain', a Kurama/Hiei doujin by Maririn.
Many depict the Urameshi crew in fantastical clothing or a bit too close for comfort, but since the art style is the same as many used in the official show (since, you know, she WAS the character designer...), I strongly hypothesize this is where the so-called proof comes from.
An unofficial piece by Kitayama featuring fantastical designs.
I really don't blame those, especially on the early days of the internet and infancy of web forums, for perhaps getting confused or even having things mistranslated. Even today, most works surrounding Yu Yu are sort of separate from the manga and based more on the anime. But if the anime and Kitayama aren't the culprit, then who?
Perhaps... Togashi's various interesting comments and choices regarding LGBT characters could shed some light as to why Kurama and Hiei are remembered as a couple. It's something he's come to be known for in recent years, especially due to many androgynous and transgender characters in Hunter x Hunter. The origin of this inclusion is a bit more tumultuous, though.
On page 116 of Volume 1, Togashi has an author's note describing how the manga he had pitched following Ten de Shōwaru Cupid was not Yu Yu Hakusho, but a series called 'Trouble Quartet'. Togashi described it as a sports manga featuring a homosexual cast and a crossdressing protagonist. This idea was quickly shot down by the editors at Shounen Jump, but it's implied the work had a few chapters planned out already that have never seen the light of day.
Trouble Quartet's protagonist, Otoda Gen. He looks a bit like Keiko...
The minor character of Miyuki, a (poorly written) transgender yokai who fought Yuusuke and Kuwabara on their way to save Yukina, Karasu with his sadistic excitement over Kurama, and Itsuki's infatuation with the broken mind of Sensui are all other very obvious examples throughout Yu Yu Hakusho of characters with LGBT traits. Hell, there's some more information regarding crossdressing in the final volume, but I don't care to spoil it.
...This looks like Otoda. Guess who it really is?
Anyways, that's all my speculation. In my personal, subjective opinion, Kurama and Hiei make a cute couple. It's easy to "ship" them and the fandom seems incredibly supportive of this, by and large. There was even a ceremony in November 2018 for their simulated marriage!
From kxxxh91 on Twitter, the display in Tokyo for their wedding.
The half-official nature as it was called by numerous Japanese fans on Twitter makes this pairing continue to be popular, and whether they are partners in love or in crime, I still think their dynamic is witty and always thrilling.