CLI v1.11.0 adds circleci notification slack commands, so you can script Slack setup across projects instead of clicking through each one.
circleci notification slack set --channel #ci-alertsconnects a project to a channel. Use a channel name or ID (private channels need the ID).circleci notification slack set --disable/--enablepauses or resumes notifications and keeps the channel configuration.circleci notification slack getshows the channel and notification settings. Add--jsonfor scripting.circleci notification slack deleteremoves the channel.
Your org needs the CircleCI Slack app installed. More details about each command can be found in the CircleCI CLI reference.