"Does an Illinois complex trust need to file an Illinois return..."

If the trust is still a grantor trust, then an Illinois return is not required to be filed.

See http://www.revenue.state.il.us/Businesses/TaxInformation/Income/fiduciary.htm

If the trust is a nongrantor trust, then the issue is somewhat more complicated as an Illinois appellate court case (Linn) held, generally, that taxing the income of a trust with no current connection to Illinois was unconstitutional, a decision with which the Illinois Department of Revenue most likely does not agree.

See https://www.quarles.com/publications/for-trusts-with-no-current-illinois-connection-income-tax-is-un...