In my world, size has been the largest break-point for communities.
It’s an interesting effect that occurs when a community, especially one predicated around B2C / enthusiast-level interest, achieves a size so large that it begins to display a series of the maladies you’ve laid out, all compounding. Cultural drift is probably the most apparent of them, where the original intent of the community’s formation has been abandoned or overshadowed by shifting motivations of the userbase, or natural attrition.
The one that always creeps up on me is the structural content taxonomy break point - what you have referred to as buried knowledge in the scaling taxonomy that generates too much noise. In my mental model of a community as a bonzai tree (or fruit tree, depending on the analogy), a discussion category (branch) can grow so large and active that it starts becoming a disservice to the core users who previously sustained it. The utility declines as activity spikes, and if the community manager isn’t careful, it’ll collapse from content value dilution.
The trick here is to be able to organically identify when a category should be allowed to split without shattering it’s core users. That tends to require a lot of hands-on attention.
I’ve seen communities wither because their primary set of load-bearing categories grew too big to be of any use, or what started out as a more casual “General Discussion” area metastasized and the content stopped being surfaceable.
The fix here is a little more art than science, but I like to keep an eye on how an “active” category is assessed. Classically, I’ve taken Reddit’s old definition, which is 5 or more posts per day counts as “Active”, which is where I know I’ve achieved enough of a self-sustaining core. For what constitutes “too active”, my lower threshold would be that same 5 but to the 3rd power for a medium sized community (so ~125 posts per day). I say to the third power here, because the model here is a series of ever-increasing donut-shapes, and what the human mind can conceivably interact with from memory for actors in our orbit.
One major item I’ve also seen is a break-point within an enterprise community that is knowledge-based, where the dreaded Overmoderation begins to take its toll. Having too many moderators, or moderation that is too insistent, hyper-strict, or scolding can have a suppressive effect on assimilation by new users, leading to a deeply stagnant pool of “graybeards”. Power-tripping moderation, or an over-effort to tone-police beyond civility and into dogmatic strictures can stifle a community’s vibrancy, stunt its culture, and ultimately reduce or fully eliminate its utility.
For communities of significant scale, it’s important to be transparency about moderation, to insist that moderation happen with grace and from a rehabilitative perspective rather than a punitive one, and that moderation remains a civic engagement for the community rather than one of prestige or entitlement.
On moderation gaps, my rule of thumb is to try to have 20% more moderators than I need, but make sure that they know they are empowered to identify additional potential moderators.