Integration with Nagios is a third party, developed by our community. 

It was forked to work with Rocket.Chat after version 7.4 and is compatible with Python 3. 

Publicly available at https://github.com/WhereIsF1/rocketchat-nagios

Original integration, works with Rocket.Chat up to version 7.3: https://github.com/andrefreitas/rocketchat-nagios