Are higher versions of segregated witness script non-standard? My understanding is that only version 0 is defined but other versions are considered valid to allow extensions using soft forks, but are they non-standard at the moment?
See @Pieter Wuille's comment, currently all witness programs with versions other than 0 will be rejected as non-standard. See interpreter.cpp L1447

Edit: As of v0.16.2, it appears that all version bytes will at least enter the mempool, provided the rules for version 0x0 (if used) are followed. See BIP 141 - Witness Program, BIP 143 - Restrictions on Public Key Type and policy.cpp#L232

Currently, versions bytes higher than 0 will cause the VerifyWitnessProgram function to always return true (for future softfork compatibility). Currently version bytes 1-16 are reserved for future use, but, "for any version byte from 0 to 16, the script must fail if the witness program has a CastToBool value of zero."