Why does my vintage Shimano rear derailleur come with brake style cable housing? I got a NOS Shimano RD-6400  rear derailler and it came with cable housing that I usually see on brake cable housing with that thick wire coil inside. I always thought those aren't suitable for shifting because they are designed to compress under load. What's the deal here?
I assume that you have a friction shifter?  A friction shifter is one that does not click into an indexed position. You just adjust it until the derailleur is lined up where you want it.
Assuming this is the case, the brake cable housing is what you should be using because it was the stuff that was around when those derailleurs were designed.  The compressionless housing, which is what the indexed shifters of today need, came about specifically for those shifters.
If my assumption is wrong, and your shifting is indexed, someone put the wrong stuff on there.  If it is working for you I don't think you need to rush out and change it, but as soon as the shifting gives you trouble, you know why, and it is time to change it.