I think there haven’t been any novel major insights since 2015, for your threshold of “novel” and “major”.
Notwithstanding that, I believe that we’ve made significant progress and that work on macrostrategy was and continues to be valuable. Most of that value is in many smaller insights, or in the refinement and diffusion of ideas that aren’t strictly speaking novel. For instance:
- The recent work on patient longtermism seems highly relevant and plausibly meets the bar for being “major”. This isn’t novel - Robin Hanson wrote about it in 2011, and Benjamin Franklin arguably implemented the idea in 1790 - but I still think that it’s a significant contribution. (There is a big difference between an idea being mentioned somewhere, possibly in very “hidden” places, and that idea being sufficiently widespread in the community to have a real impact.)
- Effective altruists are now considering a much wider variety of causes than in 2015 (see e.g. here). Perhaps none of those meet your bar for being “major”, but I think that the “discovery” (scare quotes because probably none of those is the first mention) of causes such as Reducing long-term risks from malevolent actors, invertebrate welfare, or space governance constitutes significant progress. S-risks have also gained more traction, although again the basic idea is from before 2015.
- Views on the future of artificial intelligence have become much more nuanced and diverse, compared to the relatively narrow focus on the “Bostrom-Yudkowsky view” that was more prevalent in 2015. I think this does meet the bar for “major”, although it is arguably not a single insight: relevant factors include takeoff speeds, whether AI is best thought of as a unified agent, or the likelihood of successful alignment by default. (And many critiques of the Bostrom-Yudkowsky view were written pre-2015, so it also isn't really novel.)