I see what you mean, there is slim difference indeed between typing @topic and typing 4-5 different individual @name mentions.. on a topic with 6 people, including yourself, it wouldn’t be crazy or particularly difficult to manually name mention everyone who participated in the topic. And that’s totally allowed.
Where it gets weird is when the topic has 20, 50, 100 people participating. Then we’d need to pop a REAL BIG warning about notifications, like we do for groups – or even disallow it entirely as too noisy. And possibly only staff could do a @topic mention, or very highly trusted users, similar to the caution we have by not allowing enormous @groupname notifications by default, unless the group owner enables it.
As long as it is off by default and has to be explicitly enabled by the site owners I don’t see much risk. I am a bit unclear where / how that would be enabled though. Per category doesn’t seem totally right?