As an example, in the 2015 client, you can't use :Raycast() like on modern clients. Instead you'll need to use an older, now-deprecated function like :FindPartOnRay().
I'd say the best way to learn more about stuff like this is to look for now-outdated scripting tutorials on YouTube, or to look at old captures of the Roblox wiki on the Internet Archive.
Edit: There's also an API reference site that tells you when stuff was added to Roblox, so that's also useful. Link to it: https://robloxapi.github.io/ref/