Wrong. The state of Illinois admitted that it practiced slavery, disguised as 'involuntary servitude' up until 1865.
Slavery existed in Illinois despite the Northwest Ordinance and the first two state constitutions because of slaves brought in when the state was under French rule and rules in the early days of statehood allowing slaveholders from other states to circumvent the law by placing slaves under forcible servitude until a certain age (35 for men, 32 for women) if they settled in Illinois. However, later laws forbade any African-Americans, slave or free, from settling in Illinois. By 1850, the US Census reported no slaves in the state. It would not appear that any such condition continued until 1865. In fact, Illinois was the first state to adopt the 13th Amendement after it was submitted to the states.