I recently upgraded our self hosted Bitbucket server to version 6.10.1. After doing so, I noticed our Slack integration had stopped working. While debugging, I found that an official Slack integration plugin had been created since I had implemented a web-hook workaround a couple years ago. I have installed the official slack Integration plugin and set it up using the keys provided by the Bitbucket Server app in our Slack app directory. For whatever reason however, I am not seeing notifications appear in our Slack channel for subscribed repositories.
Oddly enough though, when I subscribe a repo to the Slack plugin, I.E. telling it to send notifications about that repo to a specific Slack channel, in that channel I get a notification from the Bitbucket_Server bot user that the connection has been made for that repo.