In FIX.4.2 we introduced TradingSessionStatus(35=h) and its subscription/query message TradingSessionStatusRequest(35=g) to report on the state of trading for a venue or a market segment within a trading venue.
To report at the individual security level changes in status (trading halt, auction, etc.) the SecurityStatus(35=f) and the SecurityStatusRequest(35=e) messages were added to support communication of security trading state.
Both sets of messages were updated in EP97 as part of FIX.5.0SP2. Recommend adopting FIX Latest versions of these messages for any new trading venue.
UserNotification could be used, but I think that message was primarily targeting multiple user sessions over a one FIX session and works in concert with UserRequest/UserResponse and is focused on user management, not on reporting trading venue state changes to market participants.