A fundamental root cause of information clutter is "no ownership".
People are assigned to projects. The projects end (or are cancelled), the people move on and the documents remain behind to gather "dust" and become information clutter.
This is hard to prevent. The wiki vs. sharepoint doesn't address the clutter, it just shifts the technology base that's used to accumulate clutter.
Let's look at the clutter
Technical design info / architecture docs. Old ones don't matter. There's current and there's irrelevant. Wiki.
Last year's obsolete design information is -- well -- obsolete.
Meeting minutes, action items, etc. Action items become part of someone's backlog in a development sprint, or, they're probably never going to get done. Backlogs are wiki items. Everything else is history that might be interesting but usually isn't. If it didn't create a sprint backlog items, update an architecture, or solve a development problem, the meeting was probably a waste of time.
Project plans and roadmaps. The sprint backlog matters -- this is what a "plan and roadmap" aspires to be. If you have to supplement your plans with roadmaps, you probably ought to give up on the planning and just use Scrum and just keep the backlog current.
The original plan is someone's guess at project inception time, and not really very interesting to the current project team.
Organization business mgmt info - travel, budget info, headcount info, etc. This is a weird mixture of highly structured stuff (budget, organization) and unstructured stuff ("travel"?)
How much history do you need? None? Wiki at best. Financial or HR System is where it belongs. But, in big organizations, the accounting systems can be difficult and cumbersome to use, so we create secondary sources of information like a SharePoint page with out-of-date budget numbers because the real budget numbers are buried inside Oracle Financials.
Project pages with business analysis, requirements, etc. This is your backlog. Your project roadmap and your requirements and your analysis ought to be a single document. In the wiki.
History rarely matters. Someone's concept at project inception time of what the requirements are doesn't matter very much any more. What the requirements evolved to in their final form matters far more than any history. This is wiki material.
How old is 'too old'?
I've worked with customers that have 30-year old software. The software -- obviously -- is relevant because it's in production.
The documentation, however, is all junk. The software has been maintained. It's full of change control records. The "original" specifications would have to be meticulously rewritten with each change control folded in. Since the change control documents can be remarkably pervasive, the only way to see where the changes were applied is to read the source and -- from that -- reverse engineer the current-state specification.
If we can only understand a 30-year old app by reverse engineering the source, then, chuck the 30-year old pile of paper. It's useless.
As soon as maintenance is done, the "original" specification has been devalued.
How to clean it up?
If you create the wiki page or sharepoint site, you own it forever.
When you leave, your replacement owns it forever.
Each manager is 100% responsible for every piece of information their staff creates. They have to delete things. The weak solution is to "archive" stuff. Which is just a polite way of saying "delete" without the "D-word".
Cleanup must be every manager's ongoing responsibility. If they can't remember what it is, or why they own it, they should be required (or "encouraged") to delete it. Everything unaccessed in the last two years should be archived without question. Everything 10 years old is just irrelevant history.
It's painful, and it doesn't appear to be value-creating work. After all, we work in IT. Our job is to "write" software, not delete it. No one will do it unless compelled on threat of firing.
The cost of storage is relatively low. The cost of cleanup appears higher.
How to stop the email chain?
Refuse to participate. Create a "Break the Chain" campaign focused on replacing email chains with wiki updates (or sharepoint updates).
Be sure your wiki provides links and is faster to edit than an email.
You can't force people to give up a really, really convenient solution (Email). You have to make the wiki more valuable and almost as convenient as email.
Ramp up the value on the wiki. Deprecate email chains. Refuse to respond to email chains. Refuse to accept "to do" action items through email.