Is there any option to expose an expression rule as webservice?

Currently, you can only publish Process Models as a Web Service.

forum.appian.com/.../Publishing_Process_Models_as_Web_Services.html

Is this still true in 7.9?

forum.appian.com/.../Web_APIs.html

"This means that any data that is available inside an expression can be exposed to an external system via a Web API."

Yes, that is also correct. You can design a WebAPI to retrieve data that is available inside an expression.

A quick note, this functionality is only present in Appian 7.9.X.