Theia is not a fork of vscode (even though it looks like it). It uses VSCode's code editor (Monoco) and is written from the ground up. Presumably allowing it to support extensions, that for example, vscode does not.
As an outsider to the conversation I'd say it answers it vaguely at least.
Perhaps you could ask a question less vague than the reply it incited? What is it that you want that keeps you with vscodium over other options? Or if you don't have enough information/experience of them, just what do you like about vscodium enough that you aren't particularly feeling any need to consider alternatives?
- Certain core, built-in plugins do still contact MS servers.
- Vscode has limited what extensions can do on the platform. MS own extensions can do things that others cannot.
- Theia extensions can do what oss vscode extensions cannot. Theia supports all the non-ms specific extension API's so it is fully compatible with all/most vscode extensions. But Theia goes further. Part of Theia's vision is that you could ship your own Theia, the whole platform is open.
So, Theia is a safer bet. There is no rug to pull.
Also, think about chrome <-> chromium. When Google pushes for a new API that limits what ad-blockers can do, then Chromium has a problem. Because then it becomes a fork, as upstream becomes incompatible with chromium.
Came out of beta less then a year ago. What I mean is adoption is slow (if even there), so I'm sure there is quite a lot missing.
But that's probably fine, Theia seems to be focused more on the current audience of Eclipse which is vendor tooling (chipmakers, FPGA, custom toolchains) rather than being an editor you are supposed to use by itself.
