When /gamerule sendCommandFeedback is set to false
There is no output for the command:
/datapack list
/datapack list available
/datapack list enabled
MC-140711
"sendCommandFeedback" issue with "/datapack list"
Resolved
Invalid
3
(Unassigned)
Confirmed
(Unassigned)
Minecraft 1.13.2, Minecraft 18w48a, Minecraft 18w48b, Minecraft 18w49a
Retrieved
That's what that gamerule is for. It disables command feedback of all commands. If there's a command that still sends feedback when the gamerule is disabled, that's a bug.