When you visit the global account manager (Special:CentralAuth) for the old name of a renamed user, you only get a bland "There is no global account for "XXX" error, with no indication whatsoever that the account has been renamed (random example).
See also:
T202262: Display global rename log in "Previous global account changes" (same issue from the opposite direction)
It is immensely annoying to have to track down renames when dealing with antiabuse cases (socking). That said, some part of our vanishing works partially by restricting the display of renames. (I do not think that is sufficient to stop an improvement like this, but I am really biased on the point.)
Note the old username may be reused by a different user.
That's fine. Plain text or linked when mentioning the old name, or whatever.
some part of our vanishing works partially by restricting the display of renames
English Wikipedia's Oversight policy explicitly states rename log and move log caused by renames must not be redacted. In addition we have no easy way to redact each local rename log in potentially hundreds of wikis caused by a global rename.