I’ve been thinking about this more and I wanted to add something. When I taught university, I did do trigger warnings, in the form of my syllabus. I gave everyone a heads up as to what topics we were going to cover and gave them time and space to prepare for those topics. For me, it was about making sure my students knew I respected them and their lives and perhaps their past pains. Because class is a public space, I wanted to make sure they weren’t taken aback in public spaces. I didn’t teach literature, and never will (that’s not my discipline), so perhaps there are nuances.
I’m still not sure I want explicitly clear tags on novels, but more reflecting on this reinforces my ideas of exploring the themes in reviews – it’s form of kindness, part of the service that reviews provide to readers.