Yes, I believe all the messages are in plain text, and it’s up to the server not to log it.
It is possible to e2ee the message content yourself tho.
Edit: it looks like ntfy.sh specifically keeps messages cached in memory for a few hours befor discarding them. https://docs.ntfy.sh/config/
I read through the whole list, and monero was the only decent privacy recomendation I could find. Everything else was US-hosted. A lot of it was just recommendations from Apple and Google on “privacy” services they offer.
No mention of syncthing, matrix, xmpp, even with sections dedicated to those categories.