Its a new configurable feature, if you go to
Tools → Options → Text Editor → Your language (I did C#) → Code Style → Naming
In there I went to Manage Styles add camel Case (its in there but you need to add it to your selectable): go to the “+” sign, then add your rule accordingly.
Important: Close your solution and re-open it for changes to take effect.
For example, I only use camel Case for private methods. So I choose Private Method and required Style the new one I created “camel Case” and set it to Severity Suggestion (I also promoted it to the top).
The built in are all “Suggestions” too so you can also just turn off Messages.