Heathcliff and Ishmael are two characters from a game called Limbus Company, by a South Korean company called Project Moon. They are based off of Heathcliff from Wuthering Heights and Ishmael from Moby Dick, respectively. Their ship names include HeathMael, IshCliff, HeathIsh, ⛪⛈️/⛈️⛪, 78, and many others I'm probably forgetting. (Please let me know if there's any I've forgotten, I'm mostly only an active lurker on the Eng side of the fandom.)
There's many reasons why many people ship them, but a lot of it comes down to having fun! They have an interesting dynamic and nice chemistry, and their interactions tend to be a goldmine full of ship fuel for people who ship them. This is especially true of Canto V. A hundred different fans could give you a hundred different answers, but I think that's the most succient explanation I could give.