Logback emits a warning if a file named in <include/> isn't present.

Sometimes, an included configuration file is purely optional. E.g. I would want to mail a more verbose configuration to a customer who has some mysterious error where the standard logging simply isn't detailed enough.

Unfortunately, this means that in the standard case, I'll be spammed with the missing file warning, plus heaps of INFO messages. This might distract from relevant logback configuration problems.

Suggestion: extend <include/> so that it accepts <include optional="true" ... />. If an include is optional, it will not generate error messages if the requested configuration is not present.
The loading code might want to differentiate between "does not exist" (to suppress the message) and other problems (which probably should still be reported).