35 ILCS 200/20-180
In cases where general taxes levied on real property have been delinquent for a period of 20-30 years, the taxes shall be presumed to be uncollectible. In those cases, the County Clerk and the County Collector shall enter upon the tax records in their respective offices where those taxes appear the word "Uncollectible", and shall adjust the books and records of their respective offices as provided in this Code. In cases where any installments of special assessments or special taxes levied on real property have been delinquent for a period of 30 years, the installments shall be presumed to be uncollectible. In those cases, the Collector of the municipality which levied the special assessment or special tax and the County Clerk and the County Collector shall enter upon the tax records in their respective offices where those assessments or taxes appear the word "Uncollectible" and shall adjust the books and records of their respective offices. When taxes have been designated "uncollectible" under this Section, the municipality may use any money it holds for payment of the special assessments or special taxes for improvements similar to the projects for which the moneys were collected, and for the purchase of real or personal property, in connection with those improvements.
35 ILCS 200/20-180