I just wanted to know if it is really safe to disclose : message, public key, signed hash to open world.
There's really no chance of anyone finding my private key from the signed hash ?
There is a chance. ECDSA hash signatures can be reverse engineered to produce the private key if the variables used in the signature generation is not random enough. In addition, the private keys can also be derived from the public key by a sufficiently powerful quantum computer and the luxury of the time.
With the current state of technology (and the latest Bitcoin Core) , its nothing to be worried about and there isn't more risk from this than to send a transaction.
I'm just wondering because if this feature is not possible in bitcoin-core client for segwit address, maybe it's for a security reason...
It isn't a security issue but more of the fact that there isn't any standards for signing messages with bech32 addresses yet.