The PubSub topic described in this thread and topics, is intended for use on any channel you obtain a scoped oAuth key for:
So
You’ll need a relevant Access Key from the channel owner.
The redemptions that raise a message in chat do broadcast in the clear/publically over TMI (Twitch Messaging Interface) because they raise a PRIVMSG with the data and chat message attached. (Basically the same as a (re)sub notification in chat, not all resubs appear only those that are actually shared for example).
Not all custom redemptions (or built in ones) will raise a message over TMI. So the PubSub topic covers custom redemptions currently.