Plaid's codebase has evolved significantly since 2013, with a shift away from imperative JavaScript and towards more functional programming approaches. The team initially used Underscore.js for its benefits in avoiding loops and improving type safety, but still struggled with type errors due to inconsistent data. To address this, they developed Sanctuary, a library of safe versions of unsafe Ramda functions, which provides a way to write terse and declarative programs that work correctly for all inputs using the Maybe data type. The team's adoption of functional programming has improved their codebase's overall quality and maintainability.