Is the Slavic name Snowid still in use, or is it obsolete?
I have never heard it used in real life or in historical books or literature.Snowid - Old Polish masculine name , composed of the elements Sno- ("sleep") and -wid ("to see"). So it can mean "the one who sees dreams".
Would the same hold for the name Poniat?
No sources for it as a first name. It only functioned in surnames, e.g, Poniatowski.