Eternal September
Etymology
Coined by Usenet user Dave Fischer in 1994. Each September, new university students flooded Usenet and slowly learned its norms — until September 1993, when AOL opened the floodgates to a continuous stream of newcomers who never stopped arriving. The “September that never ended” became shorthand for culture drowned by unchecked growth.
noun phrase · /ih-TUR-nuhl sep-TEM-bur/
The permanent decline in an online community’s culture when newcomers arrive faster than they can be socialized.
What it names
The moment a community stops absorbing newbies gradually and is swamped by them for good.
Example
When the niche forum hit the front page of a huge site, its regulars mourned an Eternal September.
Related idea
Eternal September is a specific cause of the decline that enshittification describes more broadly; here the corrosion comes from unassimilated newcomers rather than platform greed.