Is github1s.com safe or not ? Can we use it for private repo?

Yeah, seems pretty safe to me.

If it asks you to log into GitHub with like the GitHub "auth this app to do this" screen then your safe.

If it DIRECTLY asks your for your GitHub USERNAME and PASSWORD then exit the website and... don't visit it again.

It is a public project: conwnet/github1s, which means it can be audited in order to check its safety.

You can see making its own FS (FileSystem) in extensions/github1s/src/github1sfs.ts.

And you can check how it manages the OAuth token in extensions/github1s/src/settings-view.ts, used when fetching the file in extensions/github1s/src/util/fetch.ts