I am interested in Dhamma surrounding the choice to ban some subjects. Please note, this is not about the subjects whatever they may be, but about the choice to actively restrict enquiry and discussion,
I do know that the Buddha said that pursuing some subjects, like the unknowables - cosmos, Nibbana etc. and I believe that the rational was about it not being beneficial for the N8fp or liberation from suffering.
As for “contemplative inquiry”, no restrictions whatsoever, including the acinteyya or ungraspable topics by the way. The Buddha only warns of becoming obsessed with them out of a desire to understand them completely, but no restriction on contemplating them.
Certain kinds of “inquiries” may be more than just intellectual. For example an inquiry in sensuality or morality, might require experience. If one is not careful or wise, this experience could be harmful and could have significant negative karmic effects.
Regarding “discussion”, I would say here we have restrictions. In one sutta the Buddha says to the monks that if they’re not discussing Dhamma then they should maintain “noble silence” (possibly this silence is a reference to meditation), and in various other places he cautions against idle chatter and discussing mundane issues. They are referred to as “Animal Talk (Tiracchāna Kathā)”, 32 in number, about such topics as kings and government, wars and armies, thieves and dangers, foodstuffs, clothes, perfumes and jewellery, etc. If I remember correctly the last few of those were about other spiritual doctrines and practices, but I think what’s meant is discussing them in a chatty or gossiping manner, not for contemplative reasons.
Which, as I understand it, means that monastics should not be discussion topics variously known as engaged Buddhist action, social activism, social justice, etc.
Which a number of monastics frequently do.
da21, dn2, dn5 and others considers many types of predicting as “debasing arts”.
See dn2 for example.
In modern terms these lists would include as “debasing arts”: weather forecasting, climate change, diplomacy predictions and estimate, national security estimates, astronomy, crop forecasting, food security, earthquake prediction, military and security forecasting, infectious disease prediction.
Debasing jobs include statistician, CPA, accountant, insurance actuarial, many types of public health (in the USA the CDC - Centers for Disease Control and Prevention).