When defining the scope of a site like this one, we need to decide what is on topic and what is off topic. How can I discuss such things in Latin? It can be a pair of adjectives or some other construction to make the distinction. The best option that comes to mind is situi aptus/ineptus, but I wouldn't be surprised if there is something better.
Here are some ideas (mainly adjectives to go with rogatum or some such word) that have been proposed so far in comments and the question itself. Feel free to add any new ideas here.
- situi aptus/ineptus
- ad hoc
- adhibere/prohibere and adhibitus/prohibitus
This list contains no explanation or justification, just possibly useful ideas. If you want to elaborate on one or more idea, please write it up in a separate answer! This answer is community wiki so that anyone can edit to add a suggestion but no one gets reputation for the votes.
My submission is [situi] consentaneus:
agreeing or according with something, suited to, becoming, meet, fit, proper
The formulaic phrase consentaneum est, in this context, would mean "That is on-topic," based on its L&S meanings:
it agrees with something, it is according to reason, fitting, consistent, proper
Off-topic could be non/haud consentaneus or even, as attested in later Latin, [situi] inconsentaneus.
One of the pleasures of this site is the civility shown to those who enquire and to those who reply. In keeping with this nonjudgmental ethos I would therefore suggest:
nitēns, (present participle of niteō) brilliant, shining --to show approbation, on-topic
nītēns (present participle of deponent nītor) on one's knees, striving, making an effort --for off-topic.
Provided that it was not said out loud, or marked with a macron, the only giveaway, I suppose, would be the Accusative/Ablative situm/sitū.