Gmail displays tags like folders/categories in the sidebar. A mail can be archived but unread and every tag in the sidebar shows how many mails with that tag are unread.
Gmail list all mails, even the archived ones, when I click on a tag in the sidebar. So it's basically like having many sorted inoxes. Only mails from unknown sources land in the "real" inbox.
Almost the same here, but with Outlook @ work and Thunderbird @ home, both of which are happy to apply rules, show me counts in folders and keep my inbox clear for oddities.
I subscribe to a number of mailing lists, these are easy rules to create and I can fit processing into my day when it suits me.
Some things I drop unread into other folders as one-offs, if the situation repeats itself a few times I build a rule. classic JIT :)
One thing I would add - when there is a thread going on, I tend to keep the last email, and nuke the rest, unless someone has actually edited before replying (unusual).
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