--message-expiry-duration
is an option that you can pass when creating a new tier (in your selfhosted instance). It is equivalent to the cache-duration
for users publishing in that tier.
For instance, if you are a ntfy Pro user, your messages are cached much longer than the normal 12h (see https://ntfy.sh/v1/tiers).
The naming is a little odd. I think cache-duration
should probably be called something else.