According to the Illinois Complete Count Commission, Illinois was undercounted in the 2010 Census with only 76% participation rate. The ramifications for undercounting in Illinois in the 2020 Census include:
According to the the 2020 Census count, Illinois lost population since the 2010 Census. As a result, Illinois will lose one Congressional seat in the U.S. House of Representatives and one Electoral College vote. It likely will lose substantial federal aid as well.
2020 Census Apportionment Results, U.S. Census Bureau
U.S. Census QuickFacts Illinois, U.S. Census Bureau
Illinois Sees Second Largest Population Loss of U.S. States, Chicago Metropolitan Agency for Planning.
Further resources on Illinois' loss of population:
Where Illinois is Losing Population, Chicago Magazine, February 4, 2021.
Five Charts That Show Who's Leaving Illinois and Why, WBEZ Chicago, January 8, 2020.
Census Total Response Rates by State, U.S. Census Bureau.