The problem:
gamerule sendCommandFeedback false fails to remove command feedback from vanilla commands such as /title, /minecraft:title, /playsound and /particle.

gamerule sendCommandFeedback false used to remove all the command spam in the earlier versions of Spigot (Not 100% sure which version was spam free, but if I'd have to guess I'd say 1.8.3) and now it doesn't remove the command feedback anymore.

Couple examples:
>title Pilvinen title test
[21:32:47 INFO]: Title command successfully executed

>playsound aurinkomaa.tuuli Pilvinen 0 64 0 1 1
[21:35:44 INFO]: Played sound 'aurinkomaa.tuuli' to Pilvinen

etc.

Steps to reproduce:
1) Install a fresh Spigot with no plugins and with default settings.
2) Start the server and login.
3) Execute from the console the following command:
gamerule sendCommandFeedback false
4) Execute from the console the following command:
title <yournamehere> title test
5) You will now see in console and in the log:
"[21:32:47 INFO]: Title command successfully executed"

When in fact you should see nothing at all.

I also tried setting the following gamerules to see if they had any effect on the command feedback, but they didn't help with this problem:

gamerule commandBlockOutput false
gamerule logAdminCommands false
gamerule reducedDebugInfo true