The forward delete key (aka "delete" aka "del", but *not* the "backspace" key) does not work for the root user.
I have just created a vanilla FreeBSD 12.2-p1 (current newest) VM and if I ssh to this server (from macOS 10.14.6, if it matters) as my own user then both the forward and backward delete keys work as expected.
But if I `su` then: backspace still works, but forward delete prints a tilde and beeps.
At a BSD user group meeting, I got a workaround, which is to edit /root/.cshrc and append:
bindkey "\e[3~" delete-char
I don't know what that means or why it helps, but it does.
I think it would be rather nicer if such a commonly used key worked as expected out-of-the-box!