As far as we know, Yitzhak has been killed by Danny's ring leader of alternate personalities, Jack Lamb, but in reality, this wouldn't happen for individuals that suffer from Dissociative identity disorder. Firstly, according to TraumaDissacociation.com, the alter of a person can never really be killed, as the host who created the identities as a coping mechanism is still alive. Instead, a personality can fuse with another, which may be caused through therapy. The end goal for anyone with this condition is not about destroying identities but finding a way to integrate them as a whole or find stabilization. In doing so, it would allow the host to gain a balance between the alters instead of, like in Danny's current case, having one take full control and leading to his time loss.

Factoring this in, it would suggest that while Yitzhak may have "died" at the hands of Jack, there's a chance that it won't be the last we see of him. Alters can often become dormant if the host no longer requires them, and we did see Yitzhak's "body" when Danny went down to the basement in the barn. But while we wait to see if Danny's brutish protector returns, what's interesting is that, while it may not seem it, Danny may have just taken his first steps in doing just that, much to the surprise of his alters.