Your question can be restated: "Is there any 3rd party content that Wizards of the Coast secretly loves and wants people to use, but (nearly) no one has heard of it?"
WotC has been, and continues to be, wildly enthusiastic about promoting a vast range of third parties who endorse D&D 5E, probably more so than any previous edition.
- DMs Guild and Adventurer's League
- hundreds of articles, podcasts, and cross promotions
- thousands of videos
- DNDBeyond and Roll20
Given the great commercial success of 5E, there's no reason to imagine WotC's communications team was incompetent and let something good fall through the cracks. If WotC likes a 5E product, then they're not shy about saying so, and plenty of people hear it (ref 1 & ref 2).
As of 2018, the list of "unofficially official" 3rd party content called out withmaterial given special recognition is:
- Guild Adept program, with a lot of content
- Critical Role content on DNDBeyond, including Blood Hunter, Gunslinger, Firearm Specialist.