Are there any current plans for segwit2x/btc1 to change the block header in a backward-incompatible way, either permanently or with a signal block (e.g., "hard fork bit"), before it goes live? This is sometimes referred to as "wipeout protection."
AFAIK, btc1 will require a large signaling block. The problem is that SPV nodes won't be able to detect it because by definition they avoid downloading entire blocks. In other words, an SPV node knows nothing about block size (and can know very little about block bodies in general).
I have found nothing about this and want to confirm that the plan is really to leave the block header untouched.
If so, it looks like SPV nodes will have no way to distinguish s2x and Bitcoin chains. They will follow whichever chain has the most proof-of-work - even if that chain contains invalid block bodies. This would have serious implications for the security of funds controlled by SPV wallets.