OK, I might be wrong about the 1771-PA specifically, or it might be working by coincidence, especially with a Series A 1771 chassis.
There's a big Knowledgebase article that we followed when putting together PLC-2 retrofits and conversions (
KB 16474, TechConnect Required) and it calls out the 1771-P1, P2, and PA, as well as the 1777-P2 and -P4, as being inappropriate for use with the PLC-5.
In general, PLC-2 power supplies with Series A 1771 chassis (1771-A2, etc)were fine for PLC-2 controllers or I/O, while PLC-5 power supplies and Series B 1771 chassis (1771-A2B, etc) could be used together.
I recall running into a few instances like what you describe: "the knowledgebase says not to use them, but they work". I don't have the details about rise time, ripple, hold-up, or fault mode behavior, so I can't adjudicate which one is right.
When dealing with power supplies from the early 1980's, you can seldom justify spending a lot of research and testing time. The capacitors on the PLC-2 era hardware aren't getting any younger, while the installed base of newer hardware and its availability is high enough to make your guideline "scrap anything older than the Reagan administration".