Just an update on this old thread. I had an occasion to test a Shimano Tourney derailleur, an old Acera derailleur, and a newer Altus derailleur with a modern 9 speed drivetrain using bar-end shifters. None of the derailleurs were beyond 8-speed. The old Acera was from an era when 7 speeds was the maximum available.

Not only did every combination of derailleur-drivetrain work perfectly once adjusted, the chain seemed to fall more squarely on the old 5-8 speed triple chainrings that I was using than on many modern 9-speed cranks I've tried.

Sheldon Brown commented on this. Paraphrasing: Older derailleurs of a similar brand work perfectly with shifters of that brand since the cable throw and derailleur swing is usually standardized and backwards-compatible.