I can relate. Our group’s all time favorite NPC is literally named TBD 😅

I think pandatheist at the Bone Box Chant blog does a solid job defining it here (and expanding on it in a followup post): https://boneboxchant.wordpress.com/2019/12/21/nsr/

To quote the definition, NSR games:

• Have a GM, Weird Setting, & Living World
• Are Rules Light & Deadly (or Consequential)
• Focus on Emergent Narrative, External Interaction, & Exploration

I think all of those things (except maybe Weird Setting IMO) cover the genre well with an intentionally broad and inclusive definition. I also think looking at examples may be most illustrative and understand that it’s more a community than one particular set of traits. Yochai Gal (of Cairn fame) has a great article on this very idea: https://newschoolrevolution.com/2022/05/04/the-new-new-school-revolution .