As advised in the relevant docs related to Request Types:

• Add or remove restrictions on request types 

Currently it's not possible to raise restricted request types via email, chat, widget, and the virtual agent.

In order to support scenarios where you have both external customers and internal teams sharing a Customer Portal, you can use restricted request types to expose certain request only to internal users that you don't want to expose to the external users.

As in general the internal users could also use Slack and Chat to raise this type of requests, this limits the internal users to raise requests via the channels explained previously by the documentation.