I thought Change #11 would be simple. The icons are in a social-links block, and an RSS feed is among the supported services.

However, adding

<!-- wp:social-link {"url":"#","service":"rss"} /--> 

to the sidebar failed. I had to dig into the WordPress code itself to find the service is named "feed", not "rss". The correct syntax for the RSS feed icon is

<!-- wp:social-link {"url":"#","service":"feed"} /-->.

🙄

Hooking up the correct link to the icon is a task for later.

Update: We put an add_filter function in functions.php to put the actual feed link on the icon (Commit 38b1a45).