I'm guessing the "conversation" value in an #ActivityPub Note object is #Mastodon specific, and other implementors have adopted it as a pseudo-standard?
Did some digging and it doesn't seem well supported. Happy to use it for #nodebb but bsd.network, for example, just sends the object id with #ctxt appended, which when queried just returns the same object. #Mastodon not surprisingly doesn't send one at all.
What I'm trying to get as is it doesn't seem there is one property I can rely on, so I may just have to create my own contexts as I go.