From the Codex: By tradition most Shadowhunter family names are compound, like “Shadowhunter” itself—in this case, “shadow” + “hunter.”
So, not all of them are compound. Throughout history, many Shadowhunter families have taken on compound names to honor Jonathan Shadowhunter himself; that’s the idea. We’ve met a lot of Shadowhunters with compound names, because we’ve met a lot of Shadowhunters from regions in which compound names appear. But there are plenty of Shadowhunters without compound names as well, like Divya Joshi. (Other non-compounded surnames we’re seen: Rocio, Ke, Rosales, Maduabuchi (which means no one is God, which is much like the inscription on the sword Glorious in City of Lost Souls, Quis ut Deus—Who is like God? No one is like God. So that’s a cool connection between Rayan’s non-compound name and Glorious.)
A compound name is not a required pre-requisite of being Nephilim. There are many Shadowhunter families who don’t have compound names. There are some languages in which compound names simply aren’t a thing that exists, and there are some families that aren’t interested in imposing the convention on their names. Those families and individuals find other ways to honor Jonathan Shadowhunter and the cause of the Nephilim; their lack of compound name isn’t considered any sort of slight.