Why is x2goclient not installed on debian9?

Question

Debian

I'm trying to install:

apt install x2goclient
***
Пакеты, имеющие неудовлетворённые зависимости:
 x2goclient : Зависит: libssh-4 (>= 0.5.4-2~) но он не будет установлен
E: Невозможно исправить ошибки, у вас отложены (held) битые пакеты.

Next, I see that I have libssh-4 installed

# dpkg -s libssh-4
Package: libssh-4
Status: install ok installed
Priority: optional
Section: libs
Installed-Size: 549
Maintainer: Laurent Bigonville <[email protected]>
Architecture: amd64
Multi-Arch: same
Source: libssh
Version: 0.8.1-1~bpo9+1

Well, apt -f install doesn't help either.
What could be the problem?