I had to write quite a lot of messages to icon theme devs to add icons I was missing in XFCE with their theme. They happily added them but from users point it seemed like they needed someone to tell them which ones were missing because they just didn't know. So having a more complete list would be nice.
I had this issue back in 2013 so I have very little idea what's the current situation, didn't change my icon pack since then, however before I wrote the post I did check archive.org and it seems like last update was around 2011, so nothing's changed since then
The 0.8.90 is also wrong, since then there've been some changes - diff:
+ system-reboot The icon used for the “Reboot” item in the desktop's panel application.
+ user-available The icon used when a user on a chat network is available to initiate a conversation with.
- user-online The icon used when a user on a chat network is available to initiate a conversation with.
