As of OpenSSH 9.0, the standard openssh scp(1) client uses the SFTP protocol by default, which is not provided by OpenWrt's standard ssh server, dropbear. Invoke via scp -O to use the legacy scp/rcp protocol instead.

Some clients, e.g. GNOME Files or Cyberduck on the Mac, do not support the scp/rcp protocol at all, only SFTP, so we need to install server-side SFTP support as an additional binary in /usr/libexec/sftp-server - this can be done with:

opkg update
opkg install openssh-sftp-server

Now you can use sftp clients such as GNOME Files or Cyberduck to connect to the OpenWrt system. Note that no additional startup scripts are required since all of that is handled by dropbear which is most likely already present on your system.

If you would like to announce the service on the network using Bonjour/Zeroconf so that Cyberduck and similar clients finds it, you can install the announce package:

opkg install announce

The announce service will notice that openssh-sftp-server is present, and will announce the sftp service on the network.