Who can create/delete a custom tab that is seen across the account by all users?
Answer:
Only “administrator” user roles (and above) can indicate to make a “Custom” tab they create be a “Default” tab for all users of that account. If a "Default" tab is created for all users of that account, administrators can indicate whether or not to allow users to make edits to that tabs filters.
Only “administrator” user roles (and above) can indicate to delete a “Custom” tab that was set as a “Default” tab for all users of that account, or to update whether or not a "Default" tab can have updates performed to it by users of that account.
A deleted custom tab cannot be restored/recovered by that user. Users must always manually recreate their “Custom” tab once more.