This sounds like it is similar to "sm_psay" from the base sourcemod plugin basechat.
https://forums.alliedmods.net/showthread.php?t=56764
Code:
sm_psay <target> <message>
A messge is privately sent to the target.
say @@target message
Using standard chat, this is a shortcut for sm_psay
sm_psay_mode
Allows non-admins to use the @@alias for sm_psay. Default is off.
Set to 1 to turn on.
You do have it so that clients can disable it for themselves, a feature the one in base sourcemod lacks (afaik?). So that's good.