There is a community integration available for connecting Mattermost with Discourse:
Work has begun to support OpenID Connect, which enables one to use any OAuth 2.0 identity solution. It is planned for Mattermost Cloud and Mattermost Self-Managed E20.
An error occurred while saving the commentNicolai Winther Hjorth commented
Please. The need for authentication from external system (JomSocial on Joomla in my case) is what is keeping us from switching to Mattermost.
Being a nonprofit small website there is no way we can pay for the other editions.