BTG just announced replay protection
Bitcoin Gold Daily Update – Replay Protection fully implemented
Nov 1, 2017 | Development
In order to ensure the safety of the Bitcoin ecosystem, Bitcoin Gold has implemented full replay protection, an essential feature that protects users’ coins from being spent unintentionally.
Bitcoin Gold has just implemented a solution called SIGHASH_FORK_ID replay protection. It is an effective two-way replay protection mechanism that enforces a new algorithm to calculate the hash of a transaction so that all new Bitcoin transactions will be invalid on Bitcoin Gold blockchain and vice versa. This makes replaying transactions impossible.
https://github.com/BTCGPU/BTCGPU/pull/109What is a transaction replay?
In the context of a Bitcoin fork, a replay happens when a transaction is valid on both sides of the fork. So a transaction can be broadcast on both chains after the fork and be a valid transaction and confirm on both. This means that if you intend on sending coins on one fork, you could accidentally end up sending your coins on the other fork as well since someone else could take your transaction and replay.
What is a replay protection?
Replay protection is a safety feature that makes sure that transactions on one chain are invalid on the other chain, thus preventing transaction replay and the possible resulting loss of funds. Replay protection can be achieved in multiple ways; anything from having a blacklisted address or output type (e.g. if an output to address X is in the transaction, consider it invalid) to changing the signature scheme to changing the transaction format entirely.
https://bitcoingold.org/replay-protection-development/