“Has semiotics been adopted by any mainstream academic philosophers?”
Ironically, I think it is philosophers like Derrida who would be most pertinent to analyzing what the question seems to assume. What is a “mainstream academic philosopher”, and what lies behind the question - why is it understood that semiotics is apparently outside “mainstream”? It’s just good to clear up a lot of those underlying assumptions, to be able to discuss the matter.
Jumping ahead, with my own assumptions -
If you are looking for confirmation from people whose judgment you trust, as to whether or not it is worth taking the time to look into those texts: sort of like “cohesion” or a confirmation bias, you can probably find someone that appeals to your sense of “trustworthy” who claims, either, “It’s not legitimate theory/science”, as well as, “It is a rational and valid theory/topic of knowledge.”
If you want designation from academia, I would turn to elite academic reference materials like the Routledge Encyclopedia of Philosophy, which has large articles on Derrida, Semiotics, and everything else you mentioned. I remember being maybe 23 in my own university library, hearing how disparaging some philosophers like Chomsky and Searle had been of Derrida, impressionably assuming by hugging close to their judgment, I was in the group of people that knew better, and reading the REP entry on Derrida, which straightforwardly made the claim that Derrida was not authentically studied or understood by some philosophers who disregarded him.
Whether or not that’s true, there is a longer-term lesson that I can get behind: it doesn’t matter. Your opinion can change a lot over time as your worldview updates. If everyone tried to understand what they do not understand instead of jumping to the conclusion that it cannot make sense if they don’t understand it, I think we would see people spending less time arguing about which philosopher is good or bad, and thinking more about the whole ocean of ideas that has existed and does exist in human culture, and how they can relate to each other. I feel very strongly, an idea doesn’t have to be true or correct for you to want to learn it - true or false is a small predicate for a much larger object in the conceptual space of ideas, at large: ideas can be seen as hypothetical, in a way. You can be interested in an idea without caring if it’s a true theory or not.
I currently think about Derrida as being a type of hermeneutics - the subjective interpretation of a text, regarding what meaning it activates in the semantic networks in you. Maybe I am wrong. I actually think semiotics overlaps better work contemporary linguistics, as I think it is the study of meaning without as much natural language syntax. It could be fit in well to modern linguistics, and it probably has.