Flag away if you think something needs flagged up to the moderators. There’s no negative issue for flagging a post on any technical or moderating level unless someone is deliberately flagging posts to be disruptive. Any who flagged something isnt seen by others.
If its gone off topic context can usually be added. if its off topic to the point of not being related to the topic its in splitting it is usually a good choice.
Usually we would say that the OP of the topic has a bit more influence in this, if they think its off topic we can split it easily, if others think its off topic we may ask about moving it or getting the topic back on discussion.
more people should follow this https://freenode.net/catalysts
As mentioned you could ask for a feature request, but it doesn’t really fit well at all on this software, and if its so offtopic that hiding it wouldn’t make a difference, why not a new topic?