So, any nice topics? ;)

There are two useful public topics: https://ntfy.sh/docs/publish/#public-topics

- https://ntfy.sh/announcements for announcements (usually only release announcements) - https://ntfy.sh/stats for daily stats (right before midnight UTC)

Note that messages are only cached for 12h server-side, so you'll not see any messages in there mostly. I suggest to just subscribe and see if you like getting these messages.

Am I understand right that with the knowledge of the topic name I can also publish announcement messages into that "official" topic?

No. There are ACLs to protect them, see https://ntfy.sh/docs/config/#access-control

But for all other topics, yes. Knowing the topic means you can publish to it.