Affects Version/s: Minecraft 1.12.2
Fix Version/s: None
In my opinion, the saved toolbars are a great feature, but easily exploitable. Bad items could be transferred from a single player world to a server.
"You can only access these in creative tho..."
True, but items like command signs and command books could also be transferred to give the player server operator. It doesn't matter if the server is part creative or not.
"Just get a plugin to disable them!"
Currently, plugins that disable the saved toolbars don't exist. There are plugins that detect foreign items, but that removes all functionality for NBT'd items, spawn eggs, maps, etc.
"This isn't a bug..."
That is true, but due to it's ability to ruin a server with almost no way to stop it, might as well be an oversight.
Recreating something like this is simple. Take a sign with a command like /tellraw or /summon, save it in a toolbar, go onto a server with creative enabled (preferably one that you created), place down the sign somewhere, and click on it.