It can't access most Microsoft online services including Copilot, which happens to disable most of the features I don't want. (I understand this is both by design, as well as because Microsoft forbids unofficial forks from doing so.)

However, MS do everything they can to make plugins not work in VSCodium. And the plugin marketplaces are separate now.

Many of the popular features in VS Code are provided by plugins that are not open source and thus not provided with VSCodium.