Bob,
It is entirely possible that it came from someone's whim, someone's overhearing of a conversation where others didn't really know, or maybe from the General Notes, Job Specificaitons, Contract Documents, Engineer, etc.
But, it does exist in one code anyway. It is not mandatory in D1.1. It is mandatory in D1.8. When D1.8 is a supplement to D1.1 for special conditions, EARTHQUAKES, does it not make sense that it improves the chances of high quality structures?
Again, D1.1 is a MINIMUM standard. D1.8 improves upon that. Any person desiring to reduce the odds for catastrophe should desire to know how to accomplish that. It doesn't take that much time to make sure heat does not go over a certain temperature. They have to be measuring for minimum pre-heat and inter-pass temps anyway. Why not continue to monitor for this maximum?
I don't argue the points that js and Al made about the forgiveness of A36 for maximum temps. But, as you mainly wanted to know where this 500° (actually 550) number came from, I believe we have answered your question. Hopefully others have found this helpful as well. And again, D1.8 does not remove A36 from this restriction.
I also have that number written into most of the WPS's that I write and use in my own shop. Limits the number of WPS's I need on file for one thing. Improves chances of quality work for another. If it ever saves one life, of which I may never know the truth of the matter, it is worth it.
Have a Great Day, Brent