Most old plugins include a #define AUTHOR or #define PLUGIN with a name or Steam ID. Try searching that name.
An file is a compiled binary plugin for the GoldSrc engine (notably Counter-Strike 1.6), written in the Pawn (formerly Small) scripting language. A "decompiler" attempts to reverse this binary back into SMA source code. Key Technical Sources Amxx To Sma Decompiler
If the AMXX virtual machine can execute it, a sufficiently skilled person can decompile it. Security through obscurity is weak. Most old plugins include a #define AUTHOR or
During the compilation process, critical changes occur: critical changes occur: