Is development with Sublime active again?
Are new plugins still being developed as many look like they have not had updated in a few years?
Certainly! LSP is new and exciting. Can't help that some packages get stale, it happens with Atom and VS Code packages too, but the community here is very much alive and kicking.
Are new features being worked on to keep SAT3 up to date with the latest VSC and Atom options?
I don't think it's being approached as having to "keep up", but new stuff is certainly coming. Two dev build just in the past week: https://forum.sublimetext.com/t/dev-build-3148/32456
As for what you're missing? Well, do you feel like you're missing anything?
I find git integration in Atom and VS Code to be superficially more fancy, but GitGutter and Gitsavvy for ST are excellent and I doubt you'll be missing features there. I switch to GitUp anyway to review commits. Atom in general has more fancy UI for some other things, but I found none of them to be really useful or of high enough quality. I don't think Atom has anything else of any real value that isn't covered by ST. VS Code has the debugger of course, never had a need for that. Last time I tried their build system integration was way behind ST's.
I think you can get integrated terminals in both, and while I like the idea of that after a day or two I stop caring and just switch back to a separate terminal anyway.
The way I see it, Atom and VS Code try to integrate utilities into an IDE-like monolithic whole, whereas with Sublime Text you'd generally switch to a dedicated (best-in-class) application (eg. Terminal, SourceTree).
ST also hardly ever trades performance for features. It was already super fast before 3.0 and they decided to make it faster, and now faster still with 3148. Browser-based editors will never catch up to that.