This is not atypical of unity dev, it goes on it's way according to plan or plans yet known.
If something is removed that Some like or find useful then generally too bad for them.
(it's possible for decisions to be reversed, maybe this will, maybe not.
At some point you just move on & use what's provided or what's provided that can be modded or possibly move on from ubuntu/unity if it becomes something one no longer likes or finds interesting..
In this case the systray is still enabled (ATM), only that users can't add to as the schema is gone as is a few bits add. code (not that much really.
So as users you obviously file bugs & hope for the best but best bet is to just put it back yourselves or if still viable thru a ppa at release
Again ATM the commit or guts of can be reverted or apps can be added to the systray which is still there as before with the schema (whitelist.
Or apps can be added back directly in the current source & 'told' to be available in the systray.
Ex. - I want vlc & audacious back in tray so in the source -
unity-6.12.0daily13.02.07/panel/PanelTray.cpp line 33.
Edited the blue to reflect an array of 4, added red, built unity, works fine, vlc & aud are back in the systray
Code:
const std::array<std::string, 4> WHITELIST {{ "JavaEmbeddedFrame", "Wine", "Vlc", "Audacious" }};