Since it seems people are hesitant to stake out an answer, I would like to offer a suggestion. I didn't want to offer my own opinion because I wanted to give the community a chance to decide, but instead of offering an opinion I'll give an estimation of what the community wants based on what types of questions were closed.
With the exception of the recently closed question (which isn't even tagged with celestial-mechanics anyway), there are four celestial mechanics questions closed as off-topic. I want to look at them one at a time to try to get an idea of what the boundary is here:
This is a question about the history of the solar system--specifically what caused the irregular rotation of a single planet. Can we find similar questions that are considered on topic?
- Is the retrograde (clockwise) rotation rate of Venus speeding up or slowing down? - This question is actually asking about the present state of a planet's rotation, but the nature of this question requires knowledge of the past conditions that led to Venus's retrograde rotation, as well as present data and future projection.
- Why is the rotation rate of Venus so slow? - This question is highly upvoted and firmly about the history of the rotation of a single planet. That said, it does have a comment from 2017 with one upvote that the question should be on Astronomy, not here. However, it also has a reply from a mod that the question is on topic because it concerns planetary science. The former question is also tagged planetary science. Given our ruling on planetary science, it makes sense that these questions were allowed. However, it raises an interesting dilemma. This specific question--to the layperson--may or may not have a planetary science answer. In fact, the Uranus question too is potentially ambiguous for someone who does not have the relevant experience to determine whether planetary science factors could cause such an irregular axial tilt.
- Space and Newton laws - This question is about the orbital motion of Mercury. This is a history question involving celestial mechanics of a planet.
Taking all these factors into account, we see a slight inclination to perceive questions about the history of the orbits and rotations of planets in the solar system as off topic, but it seems to be generally accepted.
My suggestion: Leave all questions about the orbits and rotation of planets in the solar system on topic.
This is a question about how the Sun moves within the solar system. Are there similar on topic questions to this one?
My suggestion: Questions about the Sun's motion unless directly relating to space missions or planetary motion should be off-topic.
This is a question about the orbit of Mercury. Per my first suggestion above, I don't think this should have been closed.
This is a question about the Sun's motion as tracked on the celestial sphere. In addition to lending support to my second suggestion above, I think this was also ruled off topic because the celestial sphere, when not invoked in reference to the motions of artificial satellites, is really more of a concept for Astronomy than Space Exploration, in general. It is about Earth-based observation.
I suggest that all questions about the orbits and rotation of planets in the solar system be on topic, while questions about the Sun's motions should be off topic unless directly relating to space missions or planetary motion.
This would be the position most consistent with our community's determinations to date about the scope of our site.