Here's my two cents worth:
I had heard all about these deteriorating pickguards years ago, but since my guitar is more that fifty years old, I figured that I had dodged a bullet.
A couple years ago, after moving into a historic (100+ year-old) house during a very hot/humid summer, I noticed that one of the strings had broken and that there was a little rust around some of the pickup screws.
I mistakenly attributed the damage to the climatic conditions, cleaned the guitar up and put it away again.
Just recently, now living in the desert southwest, I opened the case after about a year and, lo and behold, two broken strings and a crapload of rust/green corrosion.
After some investigation and reaching out to experts, George Gruhn told me that even a decades-old instrument that has never suffered from this problem can suddenly develop it.
So there you go; I only knew half of the story.
My advice to anyone who owns one of these guitars with a nitrocellulose pickguard is to remove the damned thing (unless you play the guitar frequently and can monitor its condition).
You can also get "replica" pickguards (made out of a non-gassing modern plastic), but they are expensive as hell (one quote was around $200; now, THERE'S the ripoff).
(I just checked my '80 Guild X-500, and so far, everything is copacetic. But the Guild has a black pickguard. Does anyone know if this problem is limited to the "tortiseshell" pickguards, or can it happen to any color pickguard?).
Good luck, guys.