Gimp had the only cheap effective, backward compatible solution:

There was a setting that showed a ruler in the screen, you could scale it to match a ruler you were holding against the screen.

It disappeared after a while...

I don't think this would be necessary if folks ensured their X server's Screens' resolutions were configured correctly.

    $ xdpyinfo | grep -A 2 '^screen #'
    screen #0:
      dimensions:    1920x1080 pixels (483x272 millimeters)
      resolution:    101x101 dots per inch
    $

The all X clients would know how many pixels represented a physical measurement on the screen.

And how do you configure it? most lcd manufacturers put wrong info on labels and dmp info

One uses

    Option "DPI" "96 x 96"

in an xorg.conf or similar file; this overrides DDC from the monitor. Note, not all monitors have the same density of pixels in both directions. https://wiki.archlinux.org/index.php/Xorg#Display_Size_and_D... has more.

but my monitor does not have the real DPI the label on it's back says... configuring by DPI is moot.

i read a little more on the link you provided... and the closest to the gimp old way of doing that is putting the width and height of the viable area of the monitor in mm. think that might work rather well if it's correctly implemented.

I wasn't suggesting reading a label. One measures the visible picture in real life, e.g. ruler, and calculates the DPI. CRTs didn't have a label on the back and, anyway, one could adjust the picture size.