Yes you can still use it I think. The oF Documentation page says to “see also: gtx_closest_point”. After a quick google search, I found the glm documentation page here: GLM_GTX_closest_point: Find closest point.
So you should be able to do the following if you include the header for the function:
// in ofApp.h
// in ofApp.cpp
glm::vec3 point = glm::vec3(0.0, 500.0, 0.0);
glm::vec3 a = glm::vec3(0.0, 0.0, 0.0);
glm::vec3 b = glm::vec3(500.0, 500.0, 0.0);
glm::vec3 result = glm::closestPointOnLine(point, a, b);
I wonder if there are some other functions in the glm class that are like this one, where they can be used as stated in the oF Documentation but only after including their specific header file(s).
Edit: Yes, there are a bunch of functions like glm::closestPointOnLine() in the GTX Extensions (Experimental) category: GTX Extensions (Experimental)