addContentLengthHeader

documentation reports that addContentLengthHeader is true by default

but when you generate slim skeleton in settings.php it is set to false
if you don’t need to explicity use it
which is the right setting ?

addContentLengthHeader
When true, Slim will add a Content-Length header to the response. If you are using a runtime analytics tool, such as New Relic, then this should be disabled.
(Default: true)

If you are using a runtime analytics tool, such as New Relic, then this should be disabled.

I would keep Slims default value (= true). Disable it only if you really need it.