I believe "yes" has become less useful over the years, with flags like force and quiet solves this now.

I think rm -rf would also suppress errors in addition to confirmation messages, so presumably if you want a "soft" force...

No it doesn't.

  > rm -rf foo/bar
  rm: foo/bar: Permission denied

If foo/bar does not exist, it should be silent. Via http://pubs.opengroup.org/onlinepubs/007904975/utilities/rm.... on the -f option:

> [...] Do not write diagnostic messages or modify the exit status in the case of nonexistent operands. [...]

Yes, but that's the only error it suppresses, and that's because "not existing" isn't really an error for a tool that's trying to delete something. In my example foo/bar existed but I didn't have permissions to delete it, and as you'll see it printed out that error.