Use .htaccess to route to the conforming version
SVR2 is attached to no success criterion. W3C references it from no Understanding document, so what it documents is part of how a conformance claim holds together rather than a rule anybody passes. The requirement underneath it is the one this technique exists to enforce. Where a conforming alternate version stands in for non-conforming content, the only way to the non-conforming version must run through the conforming one. Server configuration is how you make that true rather than merely intended, since a search engine will index the broken page and send people straight to it otherwise. The published test is more forgiving than most write-ups suggest, and it is easy to be stricter than it asks. A direct request may land on the conforming version, and it may equally land on a page offering both. Either passes. One caution on the mechanics. W3C's own page names two different Apache modules in two different places, so search by behavior rather than by module name.
How we find it in an audit
Reviewers request the non-conforming URL cold, with no referrer, the way a search result or a pasted link arrives. What comes back settles it. The conforming version passes, a page linking both versions passes, and the non-conforming page served straight up does not. Then we check the rule holds for the things people forget, which is usually documents and downloads rather than pages. This part is scriptable and we script it, because a rule that works on the URL you tested and not the other forty is not a rule.
How affected users experience it
Somebody searches for your report, clicks the first result, and lands directly in the viewer that cannot be operated with a keyboard. The accessible version exists. It is linked from a page they had no reason to visit and no way to know about. From where they are standing your site simply does not work, and the effort that went into building the alternate bought them nothing, because nothing in the path they actually took ever mentioned it.
Passes vs. fails
Passes
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^https://example\.com/report [NC]
RewriteRule ^legacy-viewer /report [R=302,L]
# A cold request lands on the conforming report, which links onward for anyone who wants the viewer.Fails
# No rule at all.
# Search engines index /legacy-viewer and send visitors straight into it.This guide is our interpretation of W3C technique SVR2: Using .htaccess to ensure that the only way to access non-conforming content is from conforming content. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.