MongoDB plugin is no longer part of Zabbix agent 2 and is now available as a loadable plugin instead. List of supported MongoDB versions has been extended to 2.6-5.3.
Plugin functionality and set of supported items haven't change.
The MongoDB plugin, along with other loadable plugins which may potentially be added in the future, is stored in the new repository zabbix-agent2-plugins. The package zabbix-release now adds this repository to the user's system.
To continue monitoring MongoDB on the installations from official Zabbix packages, update zabbix-release package and install zabbix-agent2-plugin-mongodb package.
Sources are available on CDN.