JUCE 6 still uses Lucida Grande as the default font face on macOS. This hasn’t been the default system font for over 7 years and 7 major OS releases (as of OS X Yosemite, 2014). This is, of course, not a big deal, but it does mean that anything built without explicitly overriding the defaults looks very dated, and this happens to apply to many JUCE-based plugins and applications on the market. Beyond the issue of 3rd party products, this also applies to JUCE itself, for instance, the Projucer is still using this dated font as well.
An update to the current system typeface would mean that new applications developed with JUCE would automatically look more current and modern, and fit in better with the look and feel of the operating system.