Find the lightest desktop environment

I think to measure such consumption isn't going to be easy. You can measure it simply by installing it into VM and default configuration. However, when your configuration changes so does the memory consumption. You would have to do some long-term statistics for your workflow. In my eyes it will also differ based on the distribution you are using - gentoo, archlinux will probably have different results than Ubuntu, Opensuse, RH.

There is also a strategy in Linux to use all available memory. I presume you want to use the gui on system where there is not enough memory available. For that you would need to perform your own tests. You would have to see how the environment deals with low levels of memory (if it can free the used memory, can be effective on low memory systems, et.)

If you want some newer statistics for different env. you can check this Ubuntu flavors one. It does not cover your list but most of it Comparison Of Memory Usages Of Ubuntu 19.04 And Flavors In 2019 (July)

There's a Fedora Magazine blog post about doing this on Fedora 31 in December of 2019.

The results are summarized in the below table:

Cinnamon: 624 MB Memory used
GNOME:    612 MB Memory used 
KDE:      733 MB Memory used
LXDE:     318 MB Memory used 
LXQt:     391 MB Memory used
MATE:     465 MB Memory used
XFCE:     448 MB Memory used

Depending on what you mean by "desktop environment" you might include simple window managers that use less.

You might want to look at wikipedia's list of light-weight Linux distribution, specifically the window/desktop manager column.

If you want a simple mainstream disto to install with minimal memory look at Fedora's LXQT spin or Lubuntu

Unless you establish benchmarks then you are unlikely to get comparable data. Just about everything is either marketing "fluff" or else unbenchmarked, and that includes Wikipedia.

In the first instance there is nothing online that suggests your original quoted figures are likely to have change by the ~30% which would sway any decision purely based on RAM. Of everything I have seen the only thing that has any weight [sic] is a nice little comparison done by this guy. Installed a bunch of different distros and got similar results to your post. Bunch of middleweights couple of heavyweights and a couple of lightweights. Same names as yours.

In the second I wouldn't think your "seasoned citizens" (be careful, am getting there myself) would be so grateful for a lightweight DE that has a long learning curve, especially if it is tooooo long. Know what I mean?

PS

Someone else's personal look at it here. Same story.