Unclear because both urls were uploaded to to github under open source license by a Microsoft developer, and then promptly removed. I can't find it now (could be removed, could be posted in an issue) but VSCodium have instructions on how to change to MS's marketplace.

Last I read, Microsoft hadn't replied on whether it was legal or not to use their makeplace, since it was uploaded under open source. Thus again, unclear.

Edit: https://github.com/microsoft/vscode/issues/31168 The issue where the license of the marketplace is discussed.