There was an attempt to start upstreaming changes to a more centralised, ancestor repo, but this failed almost immediately. Why? Well, whoever owns that upstream will naturally want a say in, and thus will exert control over, what gets merged in. The best case scenario is that you create a whole bunch of work for yourself to conform your code whatever arbitrary requirements upstream demands, or; the worst case scenario is that you effectively cede control over what gets run on your server. But this begs the question: why doesn't upstream just pull from downstream? It's all open source, after all, and yet it almost never happens.
It's been a couple years since then and things have only gotten worse: servers are no longer accepting many (if any) community contributions anymore, instead relying on in-house developers. In a sense, these mods are no longer open source, but rather source available: the vast majority of updates come whenever the sole dev-admin has one of their code-binge days.
Anyway, I think something similar is happening here, with open source more generally, particularly with the advent of AI: any outside contribution is just creating work for you; it's probably going to have its idiosyncrasies from its author; features that you wont want to support or spend time tearing out or waiting for them to tear it out; etc. So if you want a feature, you might as well do it yourself, or get an LLM to do it since it'll probably generate something good enough, without complaint, and without much delay.
> In a sense, these mods are no longer open source, but rather source available
I understand what sense you mean, but the terms "open source" and "source available" have very clear definitions, and it has nothing to do with accepting contributions or not, only with the license the code is published under. So in a sense only about downstream effects; upstream effects are still a courtesy also with open source.