It's a serious question because so far, none have.
Edit: Some context for those asking.
Eternal September refers to a time when an online community was overrun by new participants to the detriment of that community.
When new people arrive piecemeal, like they're doing right now, they join in and participate. If they make little social mistakes, they are steered by members of the community in the direction that the community has evolved into by supplying social, language and behavioural cues.
New participants alter their behaviour and the community grows a little with the new participant. If they don't alter their behaviour, it's likely that they're removed from the community by some agreed process that has evolved over time.
If the growth is sudden, then the community will be overwhelmed by "blissful or deliberate ignorance" and the systems for cues, moderation and removal fail and the community, often drastically, changes or ceases to exist.
The reference to September is that's when new University Students would get an account on the University computer systems and join Usenet News. They'd arrive every September, there'd be a blip in adjustment and the Usenet communities would absorb the new members.
Eternal September arrived when AOL joined its bulletin board to Usenet and it completely overran everything with people from all across the AOL userbase, most of them not first year University students.
I was there when this happened, alt.best.of.internet (ABOI) was a community where I participated. One of many "new groups" it was alphabetically the first on the AOL list and it imploded. Together with Malinda McCall, I wrote the FAQ in an ultimately fruitless attempt at educating the masses.
I've seen this play out over and over again across the decades I've been online, so that's why I asked.
The ABOI FAQ is here: https://www.itmaze.com.au/articles/aboi-faq
What would be an inciting incident for an Eternal September here?
The closest equivalence in the Fediverse is Threads, but it feels like Meta has done a decent job keeping up its walled garden combined with preliminary defederation of Threads from many smaller servers. I don't know why a major company would want to use a federated entity to compete against Reddit.
The thing about this is that it's often unexpected. AOL was on nobody's radar. Threads was coming for a while. What you don't know is what might happen if someone on Threads links to this community here and a post goes viral on Threads via a link to say Instagram, or Facebook, or both. What would happen here?
Really, I'm asking the question so people spend some time considering how important this community is to them and what might be required to work in new environments.
If the influx of new members from outside the instance became a disruption, I expect that defederation would be the first option to address it. That has already been used on Lemmy several times as some instances have become a detriment to others.