Well, Scouting America's position on old uniform items, as far as I'm aware, is still that anything that was ever official is always permitted. So in my opinion, if an award (Seabadge in this this case) is still an official award, and there was an official square knot for it at one time, then just because they stopped making the square knot doesn't mean you can't wear it if you earn the award. Given that it's discontinued, I personally have no problem with wearing a reproduction. But then, I also don't have a problem with wearing higher quality unofficial reproductions of square knots that are still officially produced either. Also, Scouting America used to produce knot variants with white, black, or green backgrounds for different program uniforms because it looked better. They stopped doing that, probably because of money issues, but I wear reproduction black background knots on my Sea Scouter uniform because it looks cleaner.
As long as you aren't wearing anything you didn't earn, I wouldn't be too worried about it. With Seabadge specifially though, remember that the pin and the knot aren't supposed to both be worn together. I wear the pin on my Sea Scout uniform and the knot on my Scouts BSA uniform.
