What happens to the schemas, components, templates and pages, when the user who created them is deleted?
What care should be taken while deleting a user in CMS?
When a user is deleted from the CM it is actually marked as deleted, but still known in the system. The schemas, components, templates and pages will remain untouched and the deleted user is still visible in the history of the item.
For workflow it could be a bit different. I don't know what happens to a workflow activity if the assigned user is deleted.
Users can be recreated after deletion, creating a user with the same username as the deleted user will result in a new user with a new TCMuri.
When you delete a user in Tridion, the user information is kept but the account is flagged as deleted, so all the history around items which they created/modified is kept and still visible