A Salesforce user at Get Cloudy Consulting informs the company's Salesforce associate they have moved...

Deactivating the user is the best way to change the user’s access and free up the Salesforce license. Deactivating a user prevents the user from logging in to Salesforce and removes the user from any assigned permission sets, queues, and groups. However, the user’s record and related data are still preserved and can be reactivated if needed. Deleting the user would also free up the license, but it would also delete the user’s record and related data, which may not be desirable. Doing nothing would not change the user’s access or free up the license, and it would pose a security risk if the user still has the login credentials.