The differences and implications between deleting a user and deactivating a user explained.
Deactivating a user is a reversible action, you can choose to reactivate that same user at a later date. It also places them under the Inactive view on the Organization > Organization Members page. Inactive users are not counted for the purposes of calculating subscription costs.
Please note that neither deleting or deactivating users deletes their data (tracked time) but it is easier to find data for deactivated users. Delete user time loses its association with the person who tracked it. For this reason we recommend the deactivation of users, rather than deleting them
With deleted users you would need to use a special approach.
Deleting a user completely removes the user from the organization and is not a reversible action. In order to rejoin they need to be invited again. We recommend you use this only in cases where you’re certain the user will never need to track time in your organization again and if you don’t need to view their data in regular reports like you did while they were active in your organization, hence this warning will appear when you try to delete a user.
Re-activating users
Switch to the ‘inactive’ view on the Organization > Organization Members page to see all deactivated users. Move your mouse over the three vertical dots on the far right of the name of the user you want to reactivate.
When a dropdown arrow appears, click on it and select Activate from the menu that appears.