As far as I know, extensions are not sandboxed either on Emacs, (Neo)vim, Jetbrains IDEs.
Of course, in many cases that can make your entire setup brittle - i.e., what happens when the package author decides to change some functionality that you carefully and tightly integrated into your system? At the same time, there's enormous, unmatched flexibility for making your own rules of the game - there's nothing that comes even close. You can change a function to do things that it was never initially designed for. For example, if there's a command that lets you perform GitHub search and open results in the browser, you can advise that command to change the behavior and instead of opening the results in the browser, send that data to an LLM and display it in a text buffer. You wouldn't have to rewrite the entire command; you would only have to override a specific part of it. In Vim, you'd have to rewrite the entire function. In VSCode, you'd likely have to make a separate extension. In Emacs, you wouldn't even have to save the damn thing into a file - you can write it in a scratch buffer and immediately try it out.
Sandboxing does not come for a free, as it creates more complex development APIs and a performance hits.
Would still be nice to have the option to opt into, for example, running as a WASM isolate - given the option of a robust sandbox, some plugins will find it desirable to migrate and gain the secure badge or however isolated plugins are marked for user identification.
But There are plugins where it’s going to be too much of an uphill battle to move to that model though. I still think on balance having sandboxed plugins, however they’re implemented, would be pretty nice.
For all that the Docker ecosystem is somewhat of a mess, it seems more than adequate for this use case.
Nope, docker alone/by itself is not a sandbox, at all. Not built for that purpose, nor suitable for that purpose.