As far as I can see there is a new default for any org created after Winter'21 (which is the release being deployed by Salesforce).
Long story short: I think you cannot change it, if your org is already at Winter'21 release. Check the Email-to-Case documentation for more info:
Insert Thread ID in the Email Subject
NOTE Any org created after the Winter '21 release defaults to a new
threading behavior. Until then, agents of existing orgs can update
email templates at their convenience. Any email templates still using
thread ID after the release return an empty string.
Plus the related release note: Enhance Security with a New Threading Behavior for Email-To-Case