if you count all the projects that migrated protocols over the years then there's a fair bit of "content" in the fediverse that predates not only activitypub (what people refer to as "fediverse" today) but also some proprietary platforms like google+ (and in this specific case have outlived them)
Is it mastodon or just something custom?
Akkoma it seems
GNU Social, it predates the Fediverse itself
if you count all the projects that migrated protocols over the years then there's a fair bit of "content" in the fediverse that predates not only activitypub (what people refer to as "fediverse" today) but also some proprietary platforms like google+ (and in this specific case have outlived them)