RbxUtility is only a utility library that was used to help with various actions such as a better version of instancing. It can’t create a backdoor and it never was one. It’s an official Roblox library but has been deprecated in favour of developer-created libraries and the lack of its usage.
Don’t use the parent argument of Instance.new. If you don’t intend to set properties afterward then it’s fine, otherwise you should generally stay away from its usage.