On the Slack subject: oembed - Unfurl links as videos in Slack - Stack Overflow
Unfurling of inline videos is made possible by allowlisting specific partner domains. Currently, there is only a small number of providers which unfurl this way in Slack, such as widely adopted services like YouTube, Loom and Vimeo. Each has been added on a case by case basis. We don’t have any immediate plans to add further video unfurls such as this
However, the answer might be outdated, according to this documentation : Unfurling links in messages | Slack. You’ll still need to create and configure an app for each PeerTube instance, so it’s not really interesting.
Mattermost oEmbed plugin seems to use an hardcoded list of allowlisted websites: