Besides just OAuth authentication, this should also cover outgoing webhooks and slash commands. These also should be configurable to use a client HTTPS certificate. The referenced documentation (https://docs.mattermost.com/deployment/certificate-based-authentication.html) doesn't refer to configuring the Mattermost server to present a client certificate when initiating HTTPS connections (either to an external OAuth server such as GitLab, or to an external service responding to an outgoing webhook/slash command.)
Anonymous
Besides just OAuth authentication, this should also cover outgoing webhooks and slash commands. These also should be configurable to use a client HTTPS certificate. The referenced documentation (https://docs.mattermost.com/deployment/certificate-based-authentication.html) doesn't refer to configuring the Mattermost server to present a client certificate when initiating HTTPS connections (either to an external OAuth server such as GitLab, or to an external service responding to an outgoing webhook/slash command.)