found this in an extension description today:

Why not use the Dev Containers from VS Code? Because Microsoft Devcontainer is a proprietary extension incompatible with VS Codium.

Why it is not on VS Code Marketplace? Because Microsoft is a VS Code gatekeeper and won’t allow anyone to publish an extension if it uses Proposed API. Except Microsoft and its “partners”, of course.

Meanwhile, these API are the only way to implement an extension like Open Remote SSH or Devcontainers.

Microsoft promises to allow everyone to use these API and publish extensions based on them in the future, once these API are “stable”. But it’s been many years since Microsoft first published an extension backed by a proposed API. It has also published many other extensions supported by the same API. However, the API is “unstable” and doesn’t seem to be going to stabilise anytime soon.