The mdeploy.php standalone script (used to download and unzip plugins
into the dirroot) and the \core\update\deployer class (as a
communication bridge between the core and the mdeploy.php) was
originally designed and implemented with the assumption that it would be
eventually used for updating the Moodle core itself, too. Therefore it
was written as standalone utility without dependency on the Moodle core
libraries.

However, it never happened and there is no real demand for that. So now
there is no need to have and maintain a completely parallel solution for
common things like fetching and unzipping plugin ZIPs.

Additional reasoning for mdeploy.php was that the core is not very
reliable during the core upgrade and we could run into various troubles.
This does not seem to be that bad. We rely on a lot of core
functionality (such as output rendering, DB access etc) and plugins
deployment seems to work well (and better) with common core libraries.

So long mdeploy, and thanks for all the hard work you did for us.