I can see why you'd recall that. Links used to require disambiguation as if they were case insensitive but they were always case sensitive (with redundant disambiguation).
An unfortunate implementation detail before 5.9 coupled the link to the page's "topic reference" (not a user-facing term) which is related to the page's file path within the .doccarchive.
DocC adds disambiguation to these file paths so that the documentation archive can be hosted on servers with both case-sensitive and case-insensitive file systems. Because this disambiguation is also reflected in the "topic reference" and links were previously tied to topic references, links had to use disambiguation they didn't really need.
For example, a type and a property like this
enum Something {}
var something: Something
needed to be written as Something-swift.enum and something-swift.property to match those "topic references". If the enum link was lowercase—like something-swift.enum—it wouldn't resolve.
In 5.9, the link is decoupled from the "topic reference" spelling which means that links can leverage their case sensitivity to avoid disambiguation and that links can contain characters which aren't allowed in file paths or in URLs.