TypeScript linter warning: no-unused-variable is deprecated; but I'm not using this config

As it says, tslint deprecated that rule (more info here https://github.com/palantir/tslint/pull/3919)

Check your tslint.json, and remove the rule and the warning should disappear.

no-unused-variable is deprecated. Since TypeScript 2.9. Please use the built-in compiler checks instead.

1. Remove deprecated no-unused-variable from your or dependency tslint.json file.

2. Specify the following compiler options in your tsconfig.json file.

"compilerOptions": {
  "noUnusedLocals": true,                /* Report errors on unused locals. */
  "noUnusedParameters": true             /* Report errors on unused parameters. */
}

Not only support for no-unused-variable rule, but the whole TSLint has been deprecated in favor of typescript-eslint.

Consider migration to new linter.