In TypeScript 4.0, support was added to allow changing the type of the variable in a catch clause from any to unknown. Allowing for code like:

// @useUnknownInCatchVariables
try {
  // ...
} catch (err: unknown) {
  // We have to verify err is an
  // error before using it as one.
  if (err instanceof Error) {
    console.log(err.message);
  }
}

// @useUnknownInCatchVariables
try {
  // ...
} catch (err: unknown) {
  // We have to verify err is an
  // error before using it as one.
  if (err instanceof Error) {
    console.log(err.message);
  }
}

This pattern ensures that error handling code becomes more comprehensive because you cannot guarantee that the object being thrown is a Error subclass ahead of time. With the flag useUnknownInCatchVariables enabled, then you do not need the additional syntax (: unknown) nor a linter rule to try enforce this behavior.