My road bike had a Sun Tour rear derailleur for more than 20 years. It finally died. Now it has a Shimano, like the vast majority of bicycles in the US.
Sun Tour went out of business back in the 90s, I think. But apparently they are back again. If you're ultra into detail on this, Sheldon Brown has an article for you.
My wife's old Trek road bike still has Sun Tour derailleurs. We got that bike at the Goodwill. It was a diamond in the haystack!
ps It broke my heart to throw away that old Sun Tour. (Since it had aluminum and steel mixed, it would be hard to recycle.)