A license like GPL is very restrictive, but I wouldn't call it source available. Maybe I'm missing your point?

A well known example is Gitlab Enterprise - you can see the source code but (IANAL) you can't use it without paying them and any modifications you make become the property of Gitlab. I remember seeing similar terms around some of the proprietary Confluent Platform components.

https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/LICENS...