When will xrandr version 1.5.1 be available in Ubuntu?

Actually, unlike libXrandr.so.2, the xrandr program is far from being mission-critical. It's just an X client — an unprivileged app you could install into your home directory to avoid clobbering the system one. Here's how you could do it (as a normal, non-root user!):

cd ~/Downloads
wget https://xorg.freedesktop.org/archive/individual/app/xrandr-1.5.1.tar.xz
tar xvf xrandr-1.5.1.tar.xz
cd xrandr-1.5.1
./configure --prefix=$HOME/opt/xrandr/
make install

For the compilation to work, you have to have installed the build dependencies: namely, the following command should do it.

sudo apt build-dep x11-xserver-utils

Then you can just launch it from the install directory:

$ ~/opt/xrandr/bin/xrandr --version
xrandr program version       1.5.1
Server reports RandR version 1.5

Or you can prepend $HOME/opt/xrandr/bin to your PATH and launch it as you normally launched the system xrandr. Once you are sure it works as you need, you can replace the system /usr/bin/xrandr (maybe having backed up it), so that any other users run it by default.

If you do replace the system binary, don't forget to hold corresponding package (on Ubuntu 16.04 it's x11-xserver-utils) to prevent updates from replacing it with an (most likely) earlier version.

Let's take a look upstream at Debian:

The relevant package is x11-xserver-utils. Here is it's Package Tracking System page.

That package is maintained by the Debian X Strike Force. Here is their mailing list archive and their QA tracker

Since you are tracking a bug, the Debian Bug Tracker page for that package is also likely to be useful.

As of this writing, the most recent Debian upload was March 2018, and Launchpad indicates that is also the version in the current Ubuntu 19.10.

So...looks like there is currently not a packaged xrandr 1.5.1 at all in either Ubuntu or Debian.

If you feel you can't wait you can install xrandr from source.

Download the tar.xz file.

cd /tmp
wget https://xorg.freedesktop.org/archive/individual/app/xrandr-1.5.1.tar.xz{,.sig}

Then extract and build the source

tar xvf xrandr-1.5.1.tar.xz
cd xrandr-1.5.1/
./configure --prefix=/usr
make
sudo make install

Then check the version:

xrandr --version
xrandr program version       1.5.1
Server reports RandR version 1.5