I wonder how that category of services, which Porter provides could be called.

I would put the following services in that category:

- Porter - Cloud66 https://www.cloud66.com - Hatchbox https://hatchbox.io

They all manage infrastructure on your behalf within a larger Cloud Service Provider.

Terms I could think of are "DevOps as a Service" or "Platform Engineering as a Service".

How would you call this?

And what alternatives do you see?