Having been here since 2016, I can tell you there is definitely no such thing as a consensus on usage of content warnings on the fediverse. It's a decentralized network that doesn't belong to any one party, so by definition there is no single culture on it. Different corners have different expectations and customs.
For my part, I've always seen them as something at the discretion of the author. As the reader, you have a plethora of tools to disengage from unwanted content on Mastodon—with even better filters coming in 4.0—and harassment is not one of them. People who are arriving now have as much right to be here and bring their own culture as the ones who came before them.
