? The remote server (reh) is part of the vscode repo itself https://github.com/microsoft/vscode/tree/main/src/vs/server/... there is a vscodium and codeoss builds of it too (reh builds).
The extension that invokes it isn’t, but there are a few implementations of it that are, and I think vscodium bundles one with their build. It’s just some shell scripts that download the reh build and run it with the correct args.