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.
@Gargron I, being new, asked if people wanted CWs if I post pics of my tarantulas. Many thought it would be nice. I was going to, because why not? But the more I think about it, I don't want to make extra clicks to see pretty spooders. What I can do, for both those that definitely want to see Ts as well as those that don't, is use a hashtag.
Don't like spiders? Exclude the hashtag from your feed.
@arctangent @Gargron someone commented on one of my posts saying I should put my Ts behind a CW.
I asked if they would expect the same of dog or cat pics? What about lizards and snakes? Where is the line drawn?
They're my pets that I love and are adorable so I'm just sharing them with the affection I would with my dog. 🤷
@arctangent @Gargron post the spiders to your hearts desire! Who knows you might even form your own informal spider based Mastodon communiy!