A CONTRIBUTORS file is easy to change by anyone hosting the repository - it's useless for the purpose of verification, unless you have a toolchain to verify each change to said file. "Sign-offs by maintainers" it not useful either unless you already know who the maintainers are, and you are kept up to date (by a trusted source) when the maintainers change. This is what Radicle does, for free, when you clone a repo.