Hiroshi, take it easy - everybody can make a mistake.
Meanwhile, I'm sure that $ZIO was initially intendent for terminal devices only. Here is the oldest link I was able to find: https://cedocs.intersystems.com/documentation/cache/cache41/olr/olrvzio....
Why not use ##CLASS(%SYS.ProcessQuery) for similar purposes? It's more powerful and better documented than some "mystic" system variables and functions.
Happy writing!

I should check docs before any "quick testing": only terminal (TRM:), telnet and file current devices can provide $ZIO with non-empty values. No client/server!

Hiroshi,
May I ask you where did it ever work?

Here are the results of quick test in Studio Output window: 

w "*"_$zio_"*"
**
w "*"_$zutil(67,12,$j)_"*"
*MYPC*
w ##CLASS(%SYS.ProcessQuery).Open("P"_$j).ClientNodeName ; prefered (documented) way
MYPC
w ##CLASS(%SYS.ProcessQuery).Open("P"_$j).ClientIPAddress
10.8.3.37

Alternative program which supports CacheActiveX connection showed the same results, by the way, regardless of server's platform, whether it was UNIX/Linux or Windows.

IRIS for Windows (x86-64) 2021.1 (Build 215U) Wed Jun 9 2021 09:39:22 EDT

while results were similar in all previous Cache/IRIS versions I managed to run.