It wasn't really "removed", it was never there. The file mysql_plugin.1 is not part of the mysql source tree. I believe it's generated from the manual (which is non-GPL and generally non-free) and put in the source tarball when a new mysql release is built.

OK, understood, but the mysql_plugin.1 itself is released with GPLv2 licence, so it shouldn't be a legal issue to include that file in the mariadb's source repository, right?

ok, let me investigate

I don't know, mysql_plugin.1 doesn't really say GPLv2 anywhere, so I'm hesitant to just take it.

It looks like mysql_plugin.1 from mysql tar ball was re-licensed in mysql-5.5.31. Until mysql-5.5.30 you can fine standard GPLv2 clause right in the man page:

"This documentation is free software; you can redistribute it and/or modify it only under the terms of the GNU General Public License as published by the Free Software Foundation; version 2 of the License."

But in mysql-5.5.31 it is licensed under totally different license:
"This software and related documentation are provided under a license agreement containing restrictions on use and disclosure and are protected by intellectual property laws. Except as expressly permitted in your license agreement or allowed by law, you may not use, copy, reproduce, translate, broadcast, modify, license, transmit, distribute, exhibit, perform, publish, or display any part, in any form, or by any means."

That was not nice from "MySQL upstream". Anyway, I guess we could still use the version from mysql-5.5.30.