Eh? I've run the OSS builds of vscode and had no trouble using the extensions marketplace. The only difference is a few specific extensions like remote code, the telemetry destinations, and the branding material.
You may not have trouble using the VSCode extensions marketplace, but it's technically against the license to use it from anything that isn't the official VSCode.
Not just technically, they do enforce this through legal action.
That's a simple piece of software, and I doubt it would make a difference if it were open sourced: it would still run on their servers under their control.