@kirby shitposter.club and it's sister sites like the sealion one started in at least 2016, which is before Mastodon.

Indieweb mentions this as well:
https://indieweb.org/GNU_social#Examples

This was set up a few months before the infamous 2016 Leslie Jones Twitter Meltdown, which made the frontpage of Time Magazine, led to Milo being banned, and would be described as Jim later as the "shitposter holocaust" as that's when the infamous shadowbans/throttles began to be rolled out en masse (which is when people would really start to look at alternatives).
https://encyclopediadramatica.online/Fembusters/Leslie_Jones%27_Twitter_Meltdown

You can ask @Moon about the early history of GS, but much of the original community of the fedi of statusnet were "white techbros", foss nerds, and similar back when they'd be at the ground floor of anything truly new.