It is only valid in On-Prem system.
Technically, when creating a custom campaign action, you also need to create an implementation class to handle the processes of this action to make it work as you expected(for example, Send Email action and Create C4C Activity action's processing logic are completely different), which cannot be easily done via Cloud system.
You can use the open-channel campaign to achieve your business requirement.
Hello Elio Centeno,
Please refer below SAP previous thread and Blog as answer for your question.