Players can not right-click signs at spawn to run any commands, even commands like /trigger, which they would normally have access to.
I'm not convinced this is a bug, perhaps just an over-looked feature during development, but I was told when making a suggestion on the /r/MinecraftSuggestions subreddit to report this as a bug instead, so I'm putting it here to hopefully give this problem some attention.
Sorry if this has already been posted. I searched but couldn't find anything that seemed like the same thing.
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I thought about that, but I couldn't come up with any conceivable reason that would be the case. And even if it was, why wouldn't they be able to run commands that non-ops can normally run, like /trigger, /me, or /say?
I heard that now non-opped players are not able to execute all types of commands from signs. But you said that it's only in Spawn Protected Chunks right? Does it work anywhere else?
No, non-opped players definitely can run all commands through the signs (not just ones they could use in the command line) outside of spawn chunks. For example, I once built a store that used these signs to purchase, (the store had to be outside of spawn chunks) and the sign itself used a /setblock command, which ran a line of other command blocks.
Edit: this comment was directed to Tan Chin Cheern. I agree with Skylinerw. I also just tested this in 1.9.4 by deoping myself, leaving spawn, running a command through a sign. I haven't heard anything about this, so I doubt it's been fixed in any recent update.
I believe that sign clicking has been removed so that only opped players can run them. This is to protect servers, I think.