The guidelines for Show HN submissions, including what is on- or off-topic, are at https://news.ycombinator.com/showhn.html .

"Show HN is for something you've made that other people can play with. HN users can try it out, give you feedback, and ask questions in the thread."

I would guess prototypes are fine, but just "ideas" with no artifact are probably not the intended type of post.

I usually like posting about my ideas on social media and trying HN for the first time. Would love to know if idea posting is allowed in the Show category!