When I do /gamerule commandBlockOutput true, command blocks will not give an output in chat, if you are on a server. In singleplayer it gives output as expected.
How to reproduce
Download Minecraft server 17w46a from minecraft.net
Start it
Set enable-command-block to true in server.properties
Restart the server
Go into minecraft and connect to the server with ip "localhost"
Place a repeating command block down
In that command block, type any command that normally would give an output, for example "time set 1000"
As you see, no output will be logged into the chat
What I exptected to happen was
...that I would get spammed with "[@: time set to 1000]" in the chat.
I have tried with /time set 1000, /give @a stone, /clear @a stone.
Please watch this video for a demonstration