u-featured has progressed and is now implemented in various places. Collecting examples

• it's published by at least one Wordpress theme: SemPress
• bridgy publish accepts and syndicates it: Bridgy Publish to Twitter should consider u-featured fallback for u-photo for article use-case snarfed/bridgy#741
• Readers: Together
• it is supported in XRay
• https://indieweb.org/featured lists some examples from the community
  ** TODO: indiemap.org could be used to get a larger overview over pages publishing it in the wild

Issues/conflicts:

• there was an issue with featured (and other) elements potentially also being implied to be photo properties, this was resolved through a change in the parsing spec: reduce instances where photo is implied microformats2-parsing#21