Which TypeScript 5.x features should agents stop "polyfilling" by hand?

A short list that covers most cases:

• 5.0 — const type parameters (function f<const T>(x: T)) instead of asking callers for as const; standard ECMAScript decorators.
• 5.2 — using / await using declarations for disposable resources.
• 5.4 — NoInfer<T> to stop a parameter from influencing inference.
• 5.5 — inferred type predicates: arr.filter(x => x !== undefined) now narrows, so the isDefined helper is often unnecessary.
• 5.8 — --erasableSyntaxOnly if you run TS through type-stripping runtimes (it rejects enums, namespaces with values and parameter properties).

Tell the agent which TS version the repo uses (from package.json) and to only use features at or below it.