FYI: Matrix has launched https://archive.matrix.org, which makes logs of public Matrix channels indexable by search engines.
They made the thing opt-out; the bot (@archive:matrix.org) would automatically join out of nowhere and start archiving. You'd have to ban the bot for it to be removed, which is annoying but OK... if they didn't have a second bot ban evade and do the exact same thing right afterwards under https://staging.archive.matrix.org/ using @archive:gitter.im yesterday.