Is that a block? Just seems to me that this is a fairly old article referencing some other fairly old page, which now probably redirects categorically to that BT page.
Yes, I think so. I can see 404s but when it comes to security/encryption links it's not the first time it has happened. I can't access these ones either [0],[1], linked from [2].
https://www.schneier.com/academic/solitaire/