A new config setting, flFeedsInDatabase, turns it on. When it's true, the server stores its RSS feeds and its subscription list in a new files table and serves them from its own domain — your feed is at https://yourserver/users/yourname/rss.xml, the subscription list at https://yourserver/data/subs.opml, and the four feed-location settings from Monday's note aren't needed at all. When it's false (the built-in default), nothing changes — S3 publishing works exactly as before.
Turning it on for an existing server is one restart: at startup the server backfills — rebuilds every user's feed, every comments feed, and the everyone feed from the database, so the files are all there before the first request. Both of our servers, rss.chat and demo.rss.chat, made the switch today.
One thing the flag can't do for you: subscribers still point at your old S3 addresses. The fix is a redirect where the old feed domain is served. Ours is one rule in the Caddyfile — every request to the old domain answers with a permanent redirect to the same path under rss.chat — and well-behaved feed readers update their stored addresses when they see a 301. Details in install.md, which now describes the database-mode install; S3 remains documented in config.md for those who want it.