The unity7 interface is historical and new policy is typically not added to it to encourage developers to move into the new style desktop interfaces. This is explained in some detail here: The desktop interfaces. The best course of action if for telegram-desktop to add x11, and I commented as such in the upstream bug.
While there is an argument that unity7 could be updated, please keep in mind the unity7 interface was designed to work with the unity 7 desktop environment and the known systems with unity 7 as the DE used an abstract socket for X instead of a named socket in /tmp. This bug is for KDE plasma on manjaro, not unity7 so a) it doesn’t surprise me that telegram-desktop not plugging x11 doesn’t work and b) as per the desktop interfaces, the x11, wayland, desktop and desktop-legacy interfaces were specifically added to support other DEs generically.