This is not really accurate:

"This error allowed multiple versions of the same row to become
visible to queries, resulting in apparent duplicates. Since the error
is in WAL replay, it would only manifest during crash recovery or on
standby servers."

I think the idea is coming from what the second sentence below is
getting at but it may be too complex to explain in a release note:

The error causes some rows to disappear from indexes resulting in
inconsistent query results on a hot standby depending on whether
indexes are used. If the standby is subsequently activated or if it
occurs during recovery after a crash or backup restore it could result
in unique constraint violations as well.

I would consider adding something like "For the problem to occur a
foreign key from another table must exist and a new row must be added
to that other table around the same time (possibly in the same
transaction) as an update to the referenced row" That would help
people judge whether their databases are vulnerable. If they don't
have foreign keys or if they have a coding pattern that causes this to
happen regularly then they should be able to figure that out and
possibly disable them if they can't update promptly.

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