Now you can do this:

const auto myPropertiesFileOptions = juce::PropertiesFile::Options()
    .withStorageFormat (juce::PropertiesFile::StorageFormat::storeAsBinary)
    .withIgnoreCaseOfKeyNames (true);

For projects written in C++17, other ways of instantiating this object and modifying just a few selected members is too awkward in my opinion. I had to make a separate builder class with a similar interface, but I believe this Options class should just have a built-in interface like this. Many other JUCE classes already do, like a similar Options class for Font, also File, String, Rectangle, Colour to name a few off the top of my head, so I was quite surprised to discover that this struct wasn't capable of that.

It may also be appropriate to add a few jasserts and fallbacks in those methods, but since this logic is mostly implemented in PropertiesFile class, I decided not to do it.