As of Jan. 21, 2020:
"Note: You can't turn off Slackbot. If you're having trouble with too many messages or custom responses, please speak to your Workspace Admins for help."
When you are in the channel in which you want to search, just use Command+F on MacOS or Ctrl+F on other operating systems. It will pre-populate the search field with in:#<channel-name> for you.
Otherwise, you need to type it manually, which also works but less convenient.