Thank you for raising awareness of this RSS / Slack issue. As you stated in your post, other RSS feeds are working as expected. Okta Customer Support is available to work directly with you to try and identify the root cause of this issue. While we of course may also need to engage with Slack Support we would like to help get the root cause identified and resolved.
Okta asked me to open an "idea" here in response to a comment about this on Twitter. If you care about this issue, please upvote the idea: https://support.okta.com/help/ideas/viewIdea.apexp?id=0872A000000bq7t