Hey there
BurnedChipmunk, we've added Collectible to 3 Spyro trophies and the Relic trophies for the 3 Crash games, but have decided to not add them to the Hidden Gem trophy or the Sly trophies that you mentioned since they don't fit in to our current definitions of the Collectible trophy.
Feel free to respond and/or reach out to us directly if you have any questions about the definitions or would like to discuss them further. Thanks!