So this is factually false.
What did happen was that Omar was one of about a dozen people who sent letters to a judge asking for leniency in one particular case. A Somali-American man named Abdirahman Daud was one of nine men arrested in the US for attempting to leave the country and join ISIS. Daud was convicted and was looking at a potential forty year prison sentence. Omar was one of over a dozen people, mostly friends and family, who wrote letters to the judge asking for leniency. She has never called for lower prison sentences in general for would be ISIS members and has consistently condemned extremists, including in this very letter.
Omar wrote that the decades long prison sentence would effectively be a life sentence for Daud as he, and the others, would be old men by the time they were released. She instead argued for a lighter sentence which would include restorative justice. But your formulation misrepresents what that is. You say "restorative justice for Americans who attemnpt to join ISIS". In the English language, the phrase "justice for" usually indicates the person who has been wronged. That's not what's going on here. Restorative justice is a concept where offenders engage in various activities which attempt to make amends, to victims and the community, for wrongs they did. The restorative justice in this case would be for the people victimized by Daud's attempted association with terrorists. The idea is to not merely warehouse these people for some amount of time and then let them loose on society again, but instead to try and fix, or ameliorate, the harm done to the victims and also, crucially, to affect a moral change in the perpetrator. Through the restorative justice process the victim is supposed to be able to heal and the perpetrator is supposed to be reformed. Omar's stance in the Daud case wasn't born of sympathy for ISIS. She's been consistently critical of them. In fact, one of the reasons that she argued against the longer sentence was because doing so would offer a propaganda victory for ISIS by seemingly confirming their claim that the US treats Muslims harshly and doesn't want them in our society. Omar's stance on restorative justice and the possible propaganda value for ISIS of harsh sentences isn't radical, and isn't limited to Muslims such as herself. Terrorism expert Peter Bergen, a white non-Muslim who was IIRC the last western journalist to interview Osama bin Laden, has expressed similar ideas, arguing that the harsh sentences handed down for crimes such as trying to join ISIS can be counter productive by seemingly confirming ISIS propaganda about Americans and also by perhaps making loved ones reluctant to turn in people who they worry might be becoming radicalized. Bergen has also, like Omar, argued for lighter prison sentences which involve more rehabilitation and restorative justice, at least for people whose crimes merely consist of trying to join ISIS, rather than actually fighting or trying to kill anyone.