It's more like this:

Before we add `-mtu` but we don't want it anymore, we want to have single `-` to single letter flags (like `-e` or grouping) and double `--` to multiple letters (like `--mtu`)

In order to not break things we added `#mtu` to have `-mtu` still working but not shown in the usage.

We juste merge a PR adding a warning when you use a `#` flag.