I noticed FIX: update flag reason message with default value (#30026) · discourse/discourse@28b4ff6 · GitHub
So I tried again:
Compared to, for example, “Your post was flagged as off-topic: the community feels it is not a good fit for the topic, as currently defined by the title and the first post,” the reason for a custom flag is only a keyword.
Maybe the “Your post was flagged as reason.” could also be used for custom flags.
And in the version for responders: “The post was flagged as reason.”
(I would also find a default text for ‘illegal’ helpful, as this is not a custom reason.)