From the Illinois Police Institute comes this sentence:
Illinois’ population count was 12,812,508 in 2020, a drop of 18,124 since 2010, according to new data released April 26 by the U.S. Census Bureau.
That is the numbers that led to the loss of a congressional seat.
But the shifts of where people live within Illinois should be determining state legislative boundaries, not previous census estimates.
State Rep. Steve Reick wrote about the dangers of using random sample numbers three weeks ago.
Those estimates showed a population loss of about 258,000 people (pehaps a reader can pull the reference).