CORRECTION: These instructions apply to an Illinois resident working remotely for a California company which is the opposite of the OP's question.
You will owe Illinois income tax whether your employer withheld tax or not. An Illinois resident is taxed on worldwide income. Working remotely for a California company counts as Illinois income because you performing services in Illinois.
Your employer is required to withhold Illinois tax, if necessary, even if you did not complete Form IL-W-4.
Illinois says:
If you do not file a completed Form IL-W-4 with your employer, if you fail to sign the form or to include all necessary information, or if you alter the form, your employer must withhold Illinois Income Tax on the entire amount of your compensation, without allowing any exemptions.
See Form-IL-W-4 instructions.
Thanks for pointing this out @TomD8.
@shangt
[Edited 03/11/23 | 10:06 PST]
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