The version you requested (5.7.21-0ubuntu0.16.04.1) is not available, but a newer one is (5.7.25-0ubuntu0.16.04.2). We redirected you there.
This file is indexed.

/usr/share/doc/mysql-server/NEWS.Debian.gz is in mysql-server 5.7.25-0ubuntu0.16.04.2.

This file is owned by root:root, with mode 0o644.

However, this file being a binary blob, APT Browse does not store it.

Download the package archive to access the file: http://archive.ubuntu.com/ubuntu/pool/main/m/mysql-5.7/mysql-server_5.7.25-0ubuntu0.16.04.2_all.deb .