How old is the air conditioning unit? If they never knew about the filter then probably it’s never been replaced.
Document everything going forward (take pictures, keep a written log and receipts), then plan on taking them to small claims court soon after moving out to recover unjustified deposit deductions. However it’s probably also a good idea to check first with your insurance company before suing for a deductible, because you will negate their cause of action to go after the full amount. The time limit to sue over a written contract in Illinois is 10 years.

