For example, I'm dealing with a file that uses the old Curve SOP (not Curve 2.0). However, I don't know how to access its documentation.

Clicking the question mark gives me the doc of Curve 2.0, but I need that of the old Curve SOP.

Two options
Old docs in versions where curve 2.0 exists:
https://www.sidefx.com/docs/houdini/nodes/sop/curve-.html [www.sidefx.com]

h18.5 docs:
https://www.sidefx.com/docs/houdini18.5/nodes/sop/curve.html [www.sidefx.com]

edit:
I don't think this is usually the case for updated nodes