Almost everyone has experienced this from one side or the other. Someone flags a post with a custom reason. A moderator looks at it and realizes that the flag didn't need a custom reason. The moderator declines the flag with "flags should only be used to make moderators aware of content that requires their intervention."
The problem here is that the users don't understand why their flags are rejected in this scenario. The current messaging can be dispiriting. Examples:
- Declined custom mod flag on a first post/late answer review item
- How can I write correct reasons for custom flags?
At the same time, it makes sense that moderators would want to get users to stop using flags when they aren't absolutely needed. So give them a tool to do that. The current tool acts more to discourage flagging at all than to encourage correct flagging. Remember the rule of ten. If one user posts on meta to ask, there are likely nine other users that felt the same way but didn't post. Maybe some will read the meta post and adjust, but many will just decide that flagging is too confusing and do something else.
Potential Solution: add a new rejection reason. Something like "This post did not need a custom flag." It could then allow the moderator to select the correct flag that could have been used.
The user would then get a message that says something like "This post could have been flagged reason instead of getting a custom flag" where reason would be replaced with something like "Too Broad" or "Not an Answer". This would educate the user and hopefully lead to fewer custom flags in the future while keeping the user enthusiastic about normal flagging.
I'm somewhat neutral on the question of whether this should be at the level of "helpful" and "declined" or just one decline reason. I also don't care if this counts as a declined flag, although I kind of think that it shouldn't. The most important part is to get the feedback right. I do think that this should appear on the custom reason form if the last custom flag received this. That way people are more likely to see it than if it is merely under the list of flags.
Note that doing this should not clear the flags on a post. In fact, it should probably add a new one for the appropriate reason. On a post that is just short of a deletion for spam, this could push it over into deletion.