Is there any way to append info to window title in case if app is running as root? Like System Settings (as root) or smth like that.

Clients can set such title, but for the WM there's no proper (let alone portable) way to do this.

*Iff* the client exposes it's pid on the window, the WM could
a) for a local client
b) on linux
inspect /proc/pid on the uid (what does not indicate internal leverage/drop, though)

There's some security focused distro (kubos?) which indicates the uid by the deco color (and external mechanisms/patches)

For a client with a static caption, simply try runnig "foo --title 'bar (as root)'"

if you interest is to visually differentiate a window running as root set root's colors and font different from your's