The invite page is created by a Heroku Elements project https://elements.heroku.com/buttons/outsideris/slack-invite-... Its github page has a screenshot https://github.com/outsideris/slack-invite-automation

I think some communities want a custom invite page explaining what the community is about. The official signin/signup page lacks any customization options.

(Both Heroku and Slack are owned by SalesForce these days. The Element project pre-dates those acquisitions)