Comment by filiptronicek

For sure legal. Technically, it's extremely simple to change the Marketplace in VS Codium, but it is against the ToS of Microsoft's Marketplace to change it to theirs (see https://github.com/microsoft/vscode/issues/31168#issuecommen... for more details). As the linked comment also states, this is a big part of the reason why Open VSX was created and is widely used by VS Code forks.