First of all, let's agree on the definition of "growth hacking". In the recent years, the term growth hacking is used in the context of finding that one growth tip, trick, hack, tool, or tactic that will cause an explosion of growth in your business.

Growing a business requires defining a growth strategy and building a systematic, repeatable growth process, with the ultimate goal of achieving sustainable, authentic growth.

Coming to your question, technically speaking, growth principles and methodologies can be applied to any business model, not only SaaS. However, there are three reasons why it is more common to have growth discussions around technology startups.

1) To conduct meaningful growth experiments, you need enough data points. If your business is too small (e.g. you have 30 clients in total) how do you test something? Technology startups typically have bigger number of data points.

2) Having a growth process requires rapid testing of several growth ideas. If you have a brick & mortar business, it gets harder to test 10 different things a week. In a software business, this is much easier.

3) Building a growth process requires an investment. And that investment is viable if your business is suitable to achieve a scalable growth. If your business model is not a scalable model by nature, it's typically not worth that investment.

So to wrap it up, in theory, yes you can apply growth philosophy, principles, processes, methods to any business model including professional services. But in practice, it is more meaningful for certain types of business models like SaaS.