I think FindAndDelete() is one of the most useless and complicated functions in the script language. It is omitted from segwit (BIP143), but we still need to support it in non-segwit scripts. Actually, FindAndDelete() would only be triggered in some weird edge cases like using out-of-range SIGHASH_SINGLE.

Non-segwit scripts also use a FindAndDelete()-like function to remove OP_CODESEPARATOR from scriptCode. Note that in BIP143, only executed OP_CODESEPARATOR are removed so it doesn’t have the FindAndDelete()-like function. OP_CODESEPARATOR in segwit scripts are useful for Tumblebit so it is not disabled in this proposal

By disabling both, it guarantees that scriptCode serialized inside SignatureHash() must be constant

If we use a softfork to remove FindAndDelete() and OP_CODESEPARATOR from non-segwit scripts, we could completely remove FindAndDelete() from the consensus code later by whitelisting all blocks before the softfork block. The first step is to make them non-standard in the next release.