> Like many companies in the last year we've switched to using Slack to improve internal communication. [...] Since Slack doesn't offer an on-premises version, we searched for other options. We found Mattermost to be the leading open source Slack-alternative and suggested a collaboration to the Mattermost team.
I'm not really sure why it's 'GitLab Mattermost' and not (at your link) 'GitLab Nginx' et al. though.
They posted a giant list of the services they use recently.
They use a ton of services.
Likely you don't want your backup to be one of your systems and another part of the company probably uses Zoom already so it is probably easy to fail over to that.
This includes many proprietary ones, we generally choose the product that will work best for us, considering the benefits of open source, but not excluding proprietary software.
Mattermost is not part of the single application that GitLab is. There is a good integration between with GitLab and our Omnibus installer allows you to easily install it. But it is a separate application from a separate company.
I'm amazed they use Slack at all. Let alone as a fallback.