I’ve made an interesting (to me) discovery about something I’ve been working on, but I don’t know if I should post it on any of the math-related stack exchange forums since there isn’t much of a question involved. Specifically, I want to share that Classical Propositional Logic is contained in another, strictly more expressive propositional logic and a basic sketch of a proof. Is such a post allowed on math.stackexchange.com or math.overflow.com?
No. That is not suitable for this site. We are a question-and-answer site. If you want to advertise a new result, then you'll need to use some other way to get the word out -- for example, a personal blog.
If there is a concrete question, then you can post a question and answer it yourself. But the question still needs to meet all the usual quality standards.