CISA's initial email announcement reducing public-facing security products preserved RSS feeds. But the website language has been updated to drop RSS as a remaining source.
The stated reasoning ("CISA wants this critical information to get the attention it deserves and ensure it is easier to find") is ... specious at best.
Even if we stipulate that keeping the general website less noisy is a net win, they could just ... provide a different web page? And it would be trivial to also create a separate RSS feed just for the vulnerability announcements. The automation is clearly already there and would be relatively easy to modify.
The actual reasoning is different from the stated reasoning, and is left as an exercise for the reader.
Edit: Jerry is setting up an email-to-Mastodon shim here!