Templates incorrectly think they're not connected to a Whonix gateway.

Qubes-Whonix
30

  1. I haven’t tested the Electrum version out on Stretch yet to know if backports are still needed now or will be soon.

Then congrats you, you system is broken and you can be compromised. Many packets in system now installed from backports repo. Because now Debian 8 jessie named “oldstable”, and not “stable” as described in guide.

Source? Its the first time I hear that apt can be compromised just by using pinning. Unless you mean its an old package that no longer receives sec updates - that would make more sense.