Would be nice to have Insecure parameter in Slack plugin for easy integration with internal/on-premise faced Mattermost server configured to use private PKI or self-signed certificates. Something like Insecure parameter in Gitea Release plugin.
sounds like a reasonable request. If you are willing to write some of the code, the quickest way to get this implemented would be to send a PR to github.com/drone-plugins/drone-slack
or if mattermost has its own webhook API that offers features above and beyond the slack webhook API, and we want to take advantage of mattermost-only features, it may make sense to create a mattermost plugin.