Comment 9 for bug 2003866

https://bugs.mysql.com/bug.php?id=109685

Fixed as of the upcoming MySQL Server 5.7.42 / 8.0.33 releases, and here's the proposed changelog entry from the documentation team:

5.7.42:
With mysqldump, using --single-transaction required either the RELOAD or
FLUSH_TABLES privilege with mysqldump v5.7.41. This requirement now only
applies when gtid_mode=ON (defaults to OFF) and with --set-gtid-purged =
ON|AUTO (defaults to AUTO).

8.0.33:
With mysqldump, using --single-transaction required either the RELOAD or
FLUSH_TABLES privilege with mysqldump v8.0.32. This requirement now only
applies when gtid_mode=ON (defaults to OFF) and with --set-gtid-purged =
ON|AUTO (defaults to AUTO).