Closely related to this thread https://community.nodebb.org/topic/15966/non-stop-spammer-sign-ups but I'll post some thoughts here rather than scattering about.
@zandertrek Admittedly, I also have many "inactive" accounts. Always made me wonder. Some of these never ever return. Some do return every once in a blue moon. Others return more frequently. I have reached out to a couple other forum "owners". Seems eighty to ninety percent plus accounts being inactive is pretty much de'rigeur modern times.
I've discussed this with few folks that I know personally "in the real" who fall w/in these groups. Their answers:
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Big Data & Privacy Concerns. These folks no longer participate in any "social media" sites due to big data and privacy concerns. Primarily stemming from awareness of just how bad the "Big Three" are in this regard. These folks tend to be older professionals, highly knowledgeable and aware of stuff like the Snowden Documents.
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Big Government. Closely related to above but slightly different spin. Many jobs in IT nowadays involve intensive background checks and/or some level of security clearance. These folks are savvy enough to know that their electronic communications, both professional and personal may be subject to "continued and ongoing monitoring". Indeed, I took a pass on a pretty sweet gig once because those clauses hidden w/in the small print of a thirty plus page background check agreement. So don't laugh: Big Brother is watching. And he's got one hell of view.
So why then, register at all?
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Access to search functionality. Just got to be this way to protect against abuse by bots. This, I suspect, is the main one. I do this myself at various site: I passively consume but need search to research. Nothing nefarious here but yeah, sometimes I use temp email addresses cuz, let's face it, many, many sites are poorly managed and compromised w/o the site owners' awareness.
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Access to private chat functionality. Again, closely related to the first two points above. Some folks like chatting w/their friends privately. They don't use other sites for this because they trust me. They're also savvy enough to know that the moment any third party scripts are involved that the notions of any sense of privacy and/or confidentiality is delusional. Yes, I can still be served up with a warrant. Ditto my cloud provider. Not a silver bullet but at least raises the bar a bit. At least in this country.
Granted these represent a minority. So what to do about the rest? Well, I suspect most fall under one of @julian umbrellas:
@julian said in How to manage stalker accounts (users with no posts):
I think a great many people register on this forum just to see what the registration flow is like, or maybe to see if they get access to new things once they are registered.
Okay, where and what, precisely is the threat? Hmm..... 
I suppose some of those private chats could be used for nefarious purposes. I sometimes wonder if Admin accounts should have access to user's private chats but feel this would be a betrayal of confidences of the community at large and I don't want to throw the baby out with the bath water. So I just "endure" until actual problems are reported. I've not had this happen.
As for actual stalkers, I've only had one such occurrence reported. I was always suspect of this particular user from the get go so I perma banned them.
So what is left? All those inactive accounts, some greater than a year. I've thought about nuking accounts older than a year sans any logins but then I ask myself what harm they really cause? I've had accounts that were dormant for long periods, sometimes over a year, others that pop in to post only once or twice a year. Indeed, I have fallen into this category myself: There's a couple sites I used to be quite active on but nowadays it may be a year or two between logins. Nuke one of these and then you've set yourself up as "the bad guy", oppressive overlord, etc. Maybe this is what some are looking for? A reason to bitch and/or defame you? Whatever. Bottom line is that I see little downside to the live and let live approach while there is potential downside to pruning.
Just some thoughts. It is a sticky wicket to be sure.