I am not sure why this flag was turned off (it wasn't explained in my commit message in 0365dc852767ae589376a7aad1fb129738e408b0 or in my PR in #4411).
Whatever the reason, without `default_ignores` turned on, most default CI configurations will immediately fail, as they most likely vendor and cache their dependencies under `vendor`, which will cause standard to run against all the vendored gems and (most likely) fail. I think we should remove this before this feature is released.
https://github.com/rubygems/rubygems/commit/677f74be48