Hello @Andy2019 ,
As the messaging will explicitly say when selecting the "Delete the lists and contacts" option, contacts in other lists not selected for deletion won't be deleted.
One potential way to easily round up these now free-floating contacts would be to go back to an email's report. You'd want the reporting page of one of the emails that you explicitly sent to just the list(s) deleted, and select-all the contacts in the sent report for deletion.
For more info on managing lists (including deleting)
Contact dashboard overview
Email reporting overview
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