It seems evident that long ago, among the community of Wizards, there was a "Warden" singular who was responsible for Demonreach. Since that day, the actual duty has apparently fallen into general obscurity, yet in the mean time, the community of Wizards has had a need for militant enforcers of the Laws of Magic, and that body collectively and individually bares the same name. How the Demonreach "Warden" and the enforcer "warden" came to share the same name is lost to us the reader, but it seems Demonreach is aware of the connection, as he acknowledges it...
I find it interesting that such an important facet of WC history is "lost" or "fallen into general obscurity"...
Wizards live much longer than mortals, so their tendency to lose information over time should be commensurately reduced.
The "average" mortal life expectancy (even when factoring in regional fluctuations) is currently around 75-ish years, with high end outliers at a century or more.
Wizardly "top end outliers" seem to be about 400 years (Ancient Mai and any like her). If the ratio above holds with Wizards of the "average" being at roughly 3/4ths of the top end... then Wizards should have an average life expectancy of about 300 years -- right about at 4x as long as mortals.
Merlin created DR (put the multi-temporal aspect aside for now). Arthurian Lore - including Merlin - occurred somewhere in the 250A.D. to 500A.D. timeframe. That's ~1500 years ago. For mortals that would be more than 20 generations, but for Wizards it's only about 5 to 6.
5 - 6 mortal generations would be roughly the time of the American Revolution or shortly afterwards.
Why would Wizards, notorious information gathers and hoarders, "forget" something like DR - or anything about it. We haven't forgotten big events from the Revolutionary period.
Seems to me that someone's hiding information about this - which, of course, Wizards are also known for. Rashid and Eb both seemed shocked and a little disturbed when they found Harry had done a Sanctum Invocation there... I presume they were aware of DR - what it is for, etc. - and of what Harry had actually done...
Maybe it's only Senior Council who knows... "need to know only" and all that...
I'm more of the opinion that instead of "losing" knowledge re DR, they've deliberately separated themselves from it for some odd reason. There hasn't been a singular Warden in a long time because they haven't wanted there to be.
I'm starting to ramble, so I'll stop... but I think the idea might have some merit... I just don't have any good data to base it on.
/sigh