This is an interesting idea, but I am slightly skeptical if it would be the most practical way to address the issue. The potential benefits are there and were discussed above, so I’ll speak about potential downsides:
To answer a non-trivial question, I usually need to take some time to really understand the problem/model/context/… In many cases, this is the most “expensive” operation. Once I’ve built that context I can often quickly answer all the questions in the topic (if they are at least a bit related). The same applies when the user asks a followup question after my first answer: I’ve already understood the problem once, I can usually quickly refresh my understanding and answer without paying the cost of digging through the question again. I also feel a sense of ownership over the topic as I know others are less likely to engage, once they see someone else has started handling it.
If people separate their questions into multiple topics, they would often/sometimes get answered by multiple people, so each would pay the cost of initial understanding. To the extent splitting the topic brings new answerers, it also puts heavier burden of the collective mental resources of answerers. And from my monitoring, we don’t have much spare answering resources - we just barely manage to stay on top of all the topics.
While I think it might be useful to remind people to not ask multiple unrelated questions at once, I wouldn’t really push for strict “one question per topic”.
Even if we wanted to support “one question per topic”, could we achieve that? The problem with notes and other written instructions is that people most likely to both read and follow written instructions AND correctly evaluate if several questions are (un)related are also quite likely to carefully think about their topics and separate their questions into topics sensibly even without explicit instructions.
From my experience the most problematic bundles of several questions are usually the less well-written questions that often ignore the instructions we’ve already put in place (e.g. “share model code”) and so I assume they would ignore this new suggestion as well.
The issue of abandoned/fragmented topics is real, so what do I propose instead?
I think we could advertise more is that it is OK to repost a question that arised at the end of a long abandoned thread and that it is OK to tag people to get their attention. It should IMHO also be the norm that if I provided a first answer in a topic, but can’t/don’t want to answer a followup question (e.g. because I don’t have time) I write a short reply to that effect. In the best case also tagging someone else to take over (via the Who to ask list). E.g.:
Sorry, can’t followup, short on time. Feel free to repost the last question as a new topic to get more attention.
Sorry, I can’t answer that, but maybe @someotheruser can?
We might also decide to invest in really marking solutions in questions (or with tags that solution is not expected) and we could then monitor unsolved topics the way we currently monitor topics with no answers - but that would be a notable change in workflow, so not sure we could pull it off.
Also everybody on TL4 (Leaders and above) can split and merge topics, which we might want to use more if a completely divergent subtopic emerges.
Does that make sense?