Acrylic is acrylic, it’s fine. Use what you can get. People were just reacting to “plastic”, as it could have been some other kind of plastic, like PVC, which would be exceedingly dangerous.
In my experience, Proofgrade acrylic isn’t particularly special compared to cast acrylic I can get from other places. Wood is a different story.
And before someone corrects “acrylic is acrylic”, yes there is a difference between cast acrylic and extruded acrylic. Not in terms of composition or safety, but the way it’s made affects how it behaves in the laser: in particular, how stinky it is, how the cut edges and engraves look, and how it behaves when stressed or exposed to solvents.