Is there a list somewhere of the Elasticsearch significant improvements from 7.9 version to 8.7
Every release gets a blog post, you would need to read each blog post for the releases between 7.9 and 8.11.
This is the category tag for the release blog posts.
In addition there are also the release notes for the detailed view:
- Release notes | Elasticsearch Guide [7.17] | Elastic
- Release notes | Elasticsearch Guide [8.11] | Elastic
But the release blog posts will be better for a higher level overview and highlights.
It's also worth noting that the difference between 7.9 and 8.10 reflects a little over 3 years of effort from a pretty large team of full-time developers and numerous open-source contributors. I expect it's hard to get a sense of the progress that's been made in that time just from reading the release notes. IMO there's no good reason not to upgrade at this point.