Others will probably be able to give much more specific info - there have been lots of threads about this subject.
One thing I can say for sure is that 97 was not the last year for real plywood.
My 2005 Classic 30 has plywood.
I have the impression that AS have been in and out of plywood over the years for reasons best known to them. Also in any (or some?) given years, some models used
osb and others used plywood. For example, although our 2005 has plywood, others have reported that their particular model of the same year had osb.
When I decided to buy our first AS three years ago, I almost changed my mind when I learned that they still used wooden floors. I really thought for sure that such a costly trailer would have some sort of composite floor impervious to water damage.
In the end I decided to buy anyway. I haven't found any rot in ours yet - not to say there isn't any - but I do try to pay a lot of attention to the caulking and check over the whole trailer including the roof pretty much every time we take it out of storage.
No guarantee, but it has to help!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.