I believe the first day of Nintendo Cup was June 14 97.
Nintendo's guide for RGB was first published Jan 97. It includes a movedex detailing secondary effects, and lists base stats for every species. It doesn't detail level up moves, it only lists the levels when new moves are learned. It only catalogues visible overworld items, not the invisible ones. Just a vague reminder that invisible items might be found in underground passages.
Enix's RGB Pokedex was first published February 97 and lists every level up move, as well as base stats for Mew. In November 97 they published a dedicated Movedex that details every move's effects with a complete table for species that learn by level up and TM. There were also lots of unofficial guides at the time, so there were plenty of resources readily available. The early strategies that saw heavy use were easy to intuit. It was obvious that freeze was broken, and even without hard date, Blizzard's higher % chance of freezing was the kind of thing you could intuit on vibes alone. Even if a player assumed it had the same chance as Ice Beam, most were comfortable trading accuracy for base power.
Critical hits and OHKOs being tied to speed were also easy to intuit. In the 97 cup Electrode and Tauros both rose to the top quickly. Persian saw a lot of early use by American players for the same reason, you didn't need to datamine the game to realize it was hitting crits 100% of the time
