What's a trope you can read in historical romance but not in contemporary?
It's professor/student romance for me. A very popular CR that came out when I was in grad school featured a young professor and a grad student. So it's not any age gap issues but just purely it being my lived experience of being a grad student at the time with a very first hand understanding of power dynamics in that situation. And just the blurb had me quite irrationally go "nope. nope nope nope".
If the pairing was in a historical setting where a bluestocking FMC was mentored/tutored by said young professor who championed her academic pursuits against societal norms or something, I'd read it.