I’m happy to share some of the story/perspective from both the elementary and GNOME design sides (which largely align).
This page is outdated (and could be way more compelling!), but this gives a gist of the situation—which put even more simply is that a classic “systray” is a specific UI dictating API by its presentation, whereas the FreeDesktop space has been forging ahead with purposeful APIs based on function/intent instead of presentation.
http://islinuxabout.xyz/systray/